Alone but not lonely: Observational evidence that binary interaction is always required to form hot subdwarf stars

Hot subdwarfs are core-helium burning stars that show lower masses and higher temperatures than canonical horizontal branch stars. They are believed to be formed when a red giant suffers an extreme mass-loss episode. Binary interaction is suggested to be the main formation channel, but the high fraction of apparently single hot subdwarfs (up to 30%) has prompted single star formation scenarios to be proposed. If such formation scenarios without interaction were possible, that would also imply the existence of hot subdwarfs in wide binaries that have undergone no interaction. We probe the existence of these systems by analysing light curves from the Transiting Exoplanet Survey Satellite (TESS) for all known hot subdwarfs with a main sequence wide binary companion, and by searching for common proper motion pairs to spectroscopically confirmed hot subdwarfs. We find that (i) the companions in composite hot subdwarfs show short rotation periods when compared to field main sequence stars. They display a triangular-shaped distribution with a peak around 2.5 days, similar to what is observed for young open clusters. This observed distribution of rotation rates for the companions in known wide hot subdwarf binaries provides evidence of previous interaction causing spin-up. We also report (ii) a shortage of hot subdwarfs with candidate common proper motion companions, considering the frequency of such systems among progenitors. We identify only 16 candidates after probing 2938 hot subdwarfs with good astrometry. Out of those, at least six seem to be hierarchical triple systems, in which the hot subdwarf is part of an inner binary. These results suggest that binary interaction is always required for the formation of hot subdwarfs.Comment: 17 pages, 11 figures, 5 tables. Accepted for publication in A&

Pulse Timing Discovery of a Three-Day Companion to the Hot Subdwarf BPM 36430

Hot subdwarf B stars are core-helium burning objects that have undergone envelope stripping, likely by a binary companion. Using high-speed photometry from the Transiting Exoplanet Survey Satellite, we have discovered the hot subdwarf BPM 36430 is a hybrid sdBV_rs pulsator exhibiting several low-amplitude g-modes and a strong p-mode pulsation. The latter shows a clear, periodic variation in its pulse arrival times. Fits to this phase oscillation imply BPM 36430 orbits a barycenter approximately 10 light-seconds away once every 3.1 d. Using the CHIRON echelle spectrograph on the CTIO 1.5-m telescope, we confirm the reflex motion by detecting a radial velocity variation with semi-amplitude, period, and phase in agreement with the pulse timings. We conclude that a white dwarf companion with minimum mass of 0.42 Msun orbits BPM 36430. Our study represents only the second time a companion orbiting a pulsating hot subdwarf or white dwarf has been detected from pulse timings and confirmed with radial velocities.Comment: 7 pages, 5 figures, 4 tables. Accepted for publication in the Astrophysical Journa

Mysterious, variable, and extremely hot : white dwarfs showing ultra-high excitation lines : I. Photometric variability

Context. About 10% of all stars exhibit absorption lines of ultra-highly excited (UHE) metals (e.g., O viii) in their optical spectra when entering the white dwarf cooling sequence. This is something that has never been observed in any other astrophysical object, and poses a decades-long mystery in our understanding of the late stages of stellar evolution. The recent discovery of a UHE white dwarf that is both spectroscopically and photometrically variable led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere. Aims. We aim to gain a better understanding of these mysterious objects by studying the photometric variability of the whole popula tion of UHE white dwarfs, and white dwarfs showing only the He ii line problem, as both phenomena are believed to be connected. Methods. We investigate (multi-band) light curves from several ground- and space-based surveys of all 16 currently known UHE white dwarfs (including one newly discovered) and eight white dwarfs that show only the He ii line problem. Results. We find that 75+8 −13% of the UHE white dwarfs, and 75+9 −19% of the He ii line problem white dwarfs are significantly photo metrically variable, with periods ranging from 0.22 d to 2.93 d and amplitudes from a few tenths to a few hundredths of a magnitude. The high variability rate is in stark contrast to the variability rate amongst normal hot white dwarfs (we find 9+4 −2%), marking UHE and He ii line problem white dwarfs as a new class of variable stars. The period distribution of our sample agrees with both the orbital period distribution of post-common-envelope binaries and the rotational period distribution of magnetic white dwarfs if we assume that the objects in our sample will spin-up as a consequence of further contraction. Conclusions. We find further evidence that UHE and He ii line problem white dwarfs are indeed related, as concluded from their overlap in the Gaia HRD, similar photometric variability rates, light-curve shapes and amplitudes, and period distributions. The lack of increasing photometric amplitudes towards longer wavelengths, as well as the nondetection of optical emission lines arising from the highly irradiated face of a hypothetical secondary in the optical spectra of our stars, makes it seem unlikely that an irradiated late-type companion is the origin of the photometric variability. Instead, we believe that spots on the surfaces of these stars and/or geometrical effects of circumstellar material might be responsible

An extremely hot white dwarf with a rapidly rotating K-type subgiant companion: UCAC2 46706450

UCAC2 46706450 is a late-type star with an ultraviolet (UV) excess. It was considered a candidate to establish a sample of FGK stars with white dwarf (WD) companions that can be used to test binary evolution models. To verify the WD nature of the companion, UV spectroscopy was performed by Parsons et al. (2016). By a detailed model-atmosphere analysis we show that the UV source is an extremely hot WD with effective temperature $T_\mathrm{eff}$ = $105\,000\pm5000$ K, mass $M/M_\odot = 0.54\pm0.02$, radius $R/R_\odot = 0.040^{+0.005}_{-0.004}$, and luminosity $L/L_\odot= 176^{+55}_{-49}$, i.e., the compact object is just about to enter the WD cooling sequence. Investigating spectra of the cool star ($T_\mathrm{eff}$ = $4945\pm250$ K) we found that it is a K-type subgiant with $M/M_\odot = 0.8-2.4$, $R/R_\odot = 5.9^{+0.7}_{-0.5}$, and $L/L_\odot= 19^{+5}_{-5}$, that is rapidly rotating with $v \sin(i)=81$ km s$^{-1}$. Optical light curves reveal a period of two days and an o-band peak-to-peak amplitude of 0.06 mag. We suggest, that it is caused by stellar rotation in connection with star spots. With the radius we infer an extremely high rotational velocity of $v_{\mathrm{rot}}=151^{+18}_{-13}$ km s$^{-1}$, thus marking the star as one of the most rapidly rotating subgiants known. This explains chromospheric activity observed by H$\alpha$ emission and emission-line cores in CaII H and K as well as NUV flux excess. From equal and constant radial velocities of the WD and the K subgiant as well as from a fit to the spectral energy distribution we infer that they form a physical, wide though unresolved binary system. Both components exhibit similar metal abundances and show iron-group elements with slightly oversolar (up to 0.6 dex) abundance, meaning that atomic diffusion in the WD atmosphere is not yet active due to a residual, weak radiation-driven wind. (abridged)Comment: 13 pages, accepted for publication in A&

Substellar companions and the formation of hot subdwarf stars

"Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics."We give a brief review over the observational evidence for close substellar companions to hot subdwarf stars. The formation of these core helium-burning objects requires huge mass loss of their red giant progenitors. It has been suggested that besides stellar companions substellar objects in close orbits may be able to trigger this mass loss. Such objects can be easily detected around hot subdwarf stars by medium or high resolution spectroscopy with an RV accuracy at the km s(-1)-level. Eclipsing systems of Vir type stick out of transit surveys because of their characteristic light curves. The best evidence that substellar objects in close orbits around sdBs exist and that they are able to trigger the required mass loss is provided by the eclipsing system SDSS J0820+0008, which was found in the course of the MUCHFUSS project. Furthermore, several candidate systems have been discovered.Final Accepted Versio

Analysis of Two Eclipsing Hot Subdwarf Binaries with a Low Mass Stellar and a Brown Dwarf Companion

The formation of hot subdwarf stars (sdBs), which are core helium-burning stars located on the extended horizontal branch, is still not understood. Many of the known hot subdwarf stars reside in close binary systems with short orbital periods between a few hours and a few days with either M star or white dwarf companions. Common envelope ejection is the most probable formation channel. Among these, eclipsing systems are of special importance because it is possible to constrain the parameters of both components tightly by combining spectroscopic and light curve analyses. We report the discovery of two eclipsing binaries with a brown dwarf (< 0.07 M*) and a 0.15 M* late main sequence star companion in close orbits around sdB stars.Comment: Part of PlanetsbeyondMS/2010 proceedings http://arxiv.org/html/1011.660

Revealing the True Nature of Hen 2-428

The nucleus of Hen 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. We present preliminary results of a thorough analysis of this interesting system, which combines quantitative non-local thermodynamic (non-LTE) equilibrium spectral modelling, radial velocity analysis, multi-band light curve fitting, and state-of-the art stellar evolutionary calculations. Importantly, we find that the dynamical system mass that is derived by using all available He II lines does not exceed the Chandrasekhar mass limit. Furthermore, the individual masses of the two central stars are too small to lead to an SN Ia in case of a dynamical explosion during the merger process.Instituto de Astrofísica de La Plat

Looking into the cradle of the grave : J22564–5910, a potential young post-merger hot subdwarf

Context. We present the discovery of J22564–5910, a new type of hot subdwarf (sdB) which shows evidence of gas present in the system and it has shallow, multi-peaked hydrogen and helium lines which vary in shape over time. All observational evidence points towards J22564–5910 being observed very shortly after the merger phase that formed it. Aims. Using high-resolution, high signal-to-noise spectroscopy, combined with multi-band photometry, Gaia astrometry, and TESS light curves, we aim to interpret these unusual spectral features. Methods. The photometry, spectra, and light curves were all analysed, and their results were combined in order to support our interpretation of the observations: the likely presence of a magnetic field combined with gas features around the sdB. Based on the triple-peaked H lines, the magnetic field strength was estimated and, by using the SHELLSPEC code, qualitative models of gas configurations were fitted to the observations. Results. All observations can either be explained by a magnetic field of ∼650 kG, which enables the formation of a centrifugal magnetosphere, or a non-magnetic hot subdwarf surrounded by a circumstellar gas disc or torus. Both scenarios are not mutually exclusive and both can be explained by a recent merger. Conclusions. J22564–5910 is the first object of its kind. It is a rapidly spinning sdB with gas still present in the system. It is the first post-merger star observed this early after the merger event, and as such it is very valuable system to test merger theories. If the magnetic field can be confirmed, it is not only the first magnetic sdB, but it hosts the strongest magnetic field ever found in a pre-white dwarf object. Thus, it could represent the long sought-after immediate ancestor of strongly magnetic white dwarfs

Analyse von Sternsystemen niedriger und hoher Masse nach der Common-Envelope Phase

Als enge Doppelsterne bezeichnet man zwei gravitativ gebundene Sterne mit so geringem Abstand, dass sie in ihrer Lebenszeit mindestens einmal wechselwirken. Dies geschieht meistens durch Massentransfer in der Roten Riesen Phase. Wenn beide Sterne ähnliche Masse besitzen, beginnt ein stabiler Massentransfer über den Lagrange Punkt zwischen beiden Sternen auf den Begleiterstern, sobald der massereichere Stern sich zur Roten Riesen Phase hin entwickelt und dabei ausdehnt. Wenn der Massenunterschied in einem engen Doppelstern M_1/M_2>1.5 überschreitet, kann der Begleiter jedoch nicht die gesamte transferierte Masse aufnehmen und es bildet sich eine gemeinsame Hülle (Common Envelope) um beide Sterne. Dieses kurze Stadium wird ``Common-Envelope Phase'' genannt. Aufgrund von Reibung mit der Hülle schrumpft der Abstand beider Sterne und die dabei frei werdende Orbitalenergie kann benutzt werden, um die Hülle abzustoßen. Dabei entsteht ein sehr enges post Common-Envelope Doppelsternsystem (PCEB). Diese Phase ist äußerst wichtig für das Verständnis der Doppelsternentwicklung und dient der Erklärung einiger Phänomene von kosmologischer Bedeutung wie zum Beispiel die Supernovae vom Typ Ia. Trotz ihrer großen Bedeutung ist unser Verständnis dafür leider noch sehr begrenzt. Die Beobachtung von Doppelsternsystemen, in denen der Abstand beider Sterne im Bereich von einem Sonnenradius ist -- viel kleiner als die typische Ausdehnung eines Roten Riesen -- zeigt, dass diese Phase existieren muss. Jedoch sind die physikalischen Prozesse noch nicht verstanden. Im Moment wird diese Phase in der Modellierung nur parameterisiert. Die Modellparameter für die Effizienz von Energie- und Drehimpulstransport sind jedoch noch immer unbekannt. Diese Arbeit beschäftigt sich mit der Analyse von Systemen mit geringer Masse nach der Common-Envelope Phase sowie einem einzigartigen, massereichen Stern, einem sogenannten Hyper-Runaway Stern, der ebenfalls durch eine Common-Envelope Ejektionsphase (CEE) gegangen sein muss. Die Systeme mit niedriger Masse, die hier untersucht wurden, bestehen aus einem heißen Subdwarf Primärstern mit einem Hauptreihenstern niedriger Masse oder einem braunen Zwerg als Begleiter. Heiße Subdwarfs vom Spektraltyp B (sdBs) sind Sterne, die sich in der Phase des Kern-Helium-Brennens befinden, jedoch fast ihre gesamte Wasserstoffhülle auf dem Roten Riesenast verloren haben. Da 50% der sdBs in engen Doppelsternen gefunden wurden, kann man ihre Entstehung am besten durch Doppelsternentwicklung erklären. Besonders enge Doppelsterne mit Perioden von Stunden, in denen das Massenverhältnis beider Komponten 5:1 beträgt, müssen eine CEE Phase durchlaufen haben. Ein anderer Erklärungsvorschlag ist, dass nicht nur massearme Hauptreihensterne sondern auch Planeten oder braune Zwerge für die Entstehung des sdB verantwortlich sein könnten, wenn sie vom Stern auf dem Roten Riesenast ``verschluckt'' werden. Deshalb sind enge sdB Doppelsterne ideal, um den Einfluss von Planeten auf die Sternentwicklung zu untersuchen. Bedeckende Doppelsternsysteme bestehend aus sdB Sternen und massearmen, kühlen Begleitern (HW Virginis Systeme) sind besonders wichtig und interessant, da man in diesen die Inklination des Systems und damit auch die Massen und den Abstand beider Begleiter bestimmen kann. Diese Information ist essentiell, um die Common-Envelope Phase zu verstehen. Der erste Teil dieser Arbeit beschäftigt sich mit der Analyse und der Suche nach HW Virginis Systemen. Doppelsterne bestehend aus heißen sdB Sternen (mit Effektivtemperatur T_eff ungefähr 30000 K) mit massearmen, kühlen Begleitern (T_eff ungefähr 3000 K) zeigen charakteristische Lichtkurven, die sich durch den Reflexionseffekt auszeichnen. Dieser resultiert aus der großen Temperaturdifferenz, die bewirkt, dass der kühle Begleiter auf der dem heißen Primärstern zugewandten Seite aufgeheizt wird. Je mehr von der aufgeheizten Seite des Begleiters sichtbar ist, desto größer ist seine Helligkeit, ähnlich wie bei den verschiedenen Mondphasen. Da eine Lichtkurve die Variation der Gesamthelligkeit darstellt, bewirkt die wechselnde Helligkeit des Begleiters eine sinusförmige Variation in der Lichtkurve. Aufgrund ihrer unverwechselbaren Lichtkurve wurden viele der HW Virginis Systeme in photometrischen Durchmusterungen gefunden, die Lichtkurven von einer großen Anzahl von Sternen beobachten. Mehrere interessante enge, bedeckende Doppelsterne, die auf diese Art entdeckt wurden, wurden in dieser Arbeit durch eine kombinierte spektroskopische und photometrische Analyse untersucht. Dies erhöht die Anzahl der untersuchten HW Virginis Systeme um 40% auf 17. Dies bietet genug Statistik, um Schlüsse über die Massen- und Periodenverteilung der bedeckenden sdB PCEB Systeme zu ziehen und diese mit PCEB Systemen mit weißen Zwergen und kühlen, massearmen Begleitern, zu denen sich sdBs entwickeln werden, zu vergleichen. Unsere photometrische Untersuchung von spektroskopisch selektierten sdB Doppelsternen im MUCHFUSS (Massive Unseen Companion to Hot Faint Underluminous Stars from SDSS) Projekt mit Hilfe des Sloan Digital Sky Surveys (SDSS) erlaubte uns außerdem zum ersten Mal, den Anteil an Reflexionseffekt-Doppelsternen und substellareren Begleitern um sdB Sterne zu bestimmen. Deren Analyse zeigte, dass mehr als 5% der sdB Doppelsterne Braune Zwerg Begleiter besitzen. Dies zeigt, dass substellare Begleiter sehr wohl die Sternentwicklung beeinflussen können. Diese Doktorarbeit ist die Grundlage für das EREBOS (Eclipsing Reflection Effect Binaries from the OGLE Survey) Projekt, das wir gerade begonnen haben. Dieses Projekt hat zum Ziel 36 im OGLE (Optical Gravitational Lensing Experiment) Survey neu entdeckte HW Virginis Systeme zu untersuchen und damit das Sample zu verdreifachen, um die Rolle von Planeten auf die Sternentwicklung besser zu untersuchen und die Common-Envelope Phase besser zu verstehen. Der zweite Teil dieser Doktorarbeit beschäftigt sich mit einem einzigartigen Runaway Stern. Als Runaway Sterne werden (massereiche) Sterne bezeichnet, die ihre Entstehungsregion mit hoher Geschwindigkeit verlassen (haben). Bei den im Galaktischen Halo in geringer Zahl auftretenden massereichen Sternen muss es sich um Runaway Sterne handeln, da dort aufgrund der geringen Dichte des interstellaren Mediums keine Sternentstehung stattfindet. Diese Sterne müssen in der Scheibe entstanden sein und binnen kurzer Zeit ausgeworfen worden sein. Um Runaway Sterne mit sehr hohen Geschwindigkeiten zu erklären, wurde das Standard-Auswurfszenario in einem Doppelstern von Przybilla et al. 2009 auf enge PCEB Systeme erweitert. Dabei entwickelt sich der deutlich massereichere Primärstern schnell, was zu einer gemeinsamen Hülle aufgrund des instabilen Massentransfers führt. Nach der Common-Envelope Phase bleibt ein enges Doppelsternsystem zurück. Sobald der Primärstern in einer Kernkollapssupernova explodiert, kann sich der Begleiter mit nahezu der Orbitalgeschwindigkeit entfernen. Dabei kann die Atmosphäre des Begleiters Supernova-Auswurfmaterial akkretieren. Das bedeutet, dass solche Sterne sehr geeignet sind, um die Nukleosynthese während einer Kernkollapssupernova zu untersuchen. HD 271791 ist der einzig bekannte Runaway Stern dessen galaktische Ruhesystem-geschwindigkeit (750+-150 km/s) die Fluchtgeschwindigkeit der Galaxis übersteigt. Er wird daher als Hyper-Runaway Stern bezeichnet. Die bisherigen Studien zu diesem Stern durch Przybilla et al. 2009 haben eine Anreicherung der alpha-Prozess Elemente gezeigt. Dies weist auf die Akkretion von Supernova Auswurfmaterial hin. Da im normalen Supernova Szenario solch hohe Geschwindigkeiten nicht erreicht werden, wird ein sehr massereicher, jedoch kompakter Primärstern benötigt, höchstwahrscheinlich ein Wolf-Rayet-Stern, der entstehen sollte, wenn in der Common-Envelope Phase die Hülle abgestoßen wird. Die theoretische Untersuchung dieses Systems durch Gvaramadze 2009 hat jedoch bezweifelt, dass es möglich ist durch eine Supernova den Runaway zu solchen Geschwindigkeiten zu beschleunigen. Die vorherige Untersuchung von HD 271791 wurde nur an optischen Spektren durchgeführt, in denen nur wenige Elemente sichtbar sind. Weitere chemische Elemente können nur anhand von Ultraviolettspektren untersucht werden. Dazu wurden Beobachtungen mit dem Hubble Space Teleskop durchgeführt. Das Ziel ist es die Nukleosynthese in einer Kernkollapssupernova näher zu untersuchen, indem die Modelle für die Spektrumsyntheserechnungen für das UV erweitert werden, um in diesem Wellenlängenbereich Elementhäufigkeiten zu bestimmen. In diesem Bereich ist ein Großteil der Linien der schwereren Elemente sichtbar. Vor allem sind dort auch Linien der Elemente zu sehen, die nur durch schnellen Neutroneneinfang (r-Prozess) produziert werden können. Dieser sollte in Kernkollapssupernovae stattfinden, da dort die notwendigen hohen Neutronenflüsse auftreten. Die Spektrumsynthese wurde an vier hellen B Sternen mit verschiedenen Temperaturen von 17500-33000 K getestet, basierend auf Sternparametern, die im Optischen bestimmt wurden. Anschließend wurde HD 271791 mit einem Spektrum mit besserer Qualität im Optischen erneut untersucht, um genauere Sternparameter und Elementhäufigkeiten zu bestimmen. Es wurden auch einige Simulationen durchgeführt, um zu bestimmen, welche Geschwindigkeiten unser Objekt im Supernova Szenario erreichen kann, was Rückschlüsse auf das post Common-Envelope System zulässt. Basierend auf den stellaren Parametern, die im Optischen bestimmt wurden, konnten Elementhäufigkeiten für Elemente bis zur Eisengruppe bestimmt werden. Diese Arbeit bietet nun die Grundlage für die Bestimmung von Elementhäufigkeiten von B Sternen im UV. Es ist jetzt möglich Elementhäufigkeiten auch für die Eisengruppe und Elemente darüber hinaus zu bestimmen. Damit sollte die genauere Untersuchung der Nukleosynthese in einer Kernkollapssupernova, insbesondere der r-Prozess Elemente, in naher Zukunft möglich sein.Close binaries are two gravitationally bound stars, which orbit each other with a sufficiently small separation so that they interact with each other at least once in their lifetime. This happens mostly by mass transfer in the red giant phase. If both stars have similar masses, a stable mass transfer via the Lagrange points between them will be initiated as soon as the more massive star evolves towards the red giant branch and is expanding. In close binaries with sufficient mass difference (M_1/M_2>1.5) the companion cannot accrete all mass transfered and a common envelope around both stars is formed. Because of friction in the envelope the separation of both stars shrinks and the free orbital energy can be used to eject the envelope. The outcome is a very close binary, a post common-envelope binary system (PCEB). This phase is highly important for the understanding of binary evolution and as well as for the explanation of for example supernovae of type Ia, which are of cosmological significance. Despite this fact, our understanding of this immensely important phase is quite limited. The observation of binaries with separations of the order of a solar radius -- much smaller than the typical radius of a red giant of hundreds of solar radii -- shows that the common-envelop phase has to exist, but the physical processes are only qualitatively understood. The description of this phase is only parametrized in binary stellar evolution models at the moment. The model parameters for the efficiency of the transport of the energy and angular momentum, however, are still unknown. This thesis covers the analysis of low-mass systems as well as the analysis of a unique, massive star, a so-called hyper-runaway star, which has to have undergone a common-envelope ejection phase. The low-mass systems investigated here are very close binaries consisting of a hot subdwarf primary star and a low-mass main-sequence or brown dwarf companion. Hot subdwarf stars of spectral type B (sdB) are core-helium burning stars on the extreme horizontal branch, which lost almost their entire hydrogen envelope on the tip of the red giant branch. As about 50% of the sdBs are found in close binary systems, the best explanation for their formation is binary evolution. In particular close sdB binaries with periods of hours and with a mass ratio of 5:1 can only be explained by a previous common-envelope phase. It was also suggested that not only low-mass stars but also planets or brown dwarfs could be responsible for the mass loss, when they are engulfed in the envelope of the star. Therefore, close sdB binaries with low-mass companions are ideal to study the influence of planets on stellar evolution. Of those, eclipsing (HW Virginis systems) are in particular important, as the inclination can be determined and, hence, the masses and separation can be derived, which are essential for the understanding of the common-envelope phase. The topic of the first part of this thesis is the analysis and the search for common-envelope systems. Binaries consisting of hot subdwarf stars (with effective temperature T_eff=30000 K) with low-mass, cool companions (T_eff=3000 K) have characteristic lightcurves showing the reflection effect. This effect results from the large temperature difference between the two objects. Consequently, the cool companion is heated up on one side by the close hot primary star. The more of the heated side of the companion is visible, the higher is its luminosity similar to the different moon phases. As a lightcurve represents the time evolution of the combined luminosity of both components, the varying flux of the companion causes a sinusoidal variation in the lightcurve. Many of the HW Virginis systems have been found in photometric surveys, which observe lightcurves of a large number of stars, due to the easily recognizable lightcurve. Several eclipsing sdB binaries, which have been found in this way, were investigated in this thesis by a combined photometric and spectroscopic analysis. This increased the number of analyzed HW Virginis systems by 40% to 17 providing a sufficiently large sample to draw first conclusions about their mass and period distribution and a comparison to PCEBs with white dwarf primaries, which are the successors of the sdB binaries. Our photometric investigation of spectroscopically selected sdB binaries from the Sloan Digital Sky Survey allowed us to determine the fraction of reflection effect binaries and substellar objects around sdB stars for the first time. The analysis showed that more than 5% of the sdB binaries have substellar companions. This shows that substellar companions can indeed influence stellar evolution. This work is the basis for the EREBOS (Eclipsing Binaries from the OGLE Survey) that we just started. This project aims at studying 36 HW Virginis systems recently discovered by the OGLE (Optical Gravitational Lensing Experiment) survey in order to further investigate the role of planets on stellar evolution and develop a better understanding of the common-envelope phase. The topic of the second part of this thesis is a unique runaway star. Runaway stars are (massive) stars that are currently leaving or already have left their birth-place with high velocity. The massive stars found in low numbers in the halo have to be runaway stars, as no star formation occurs there because of the low density of the interstellar medium. Those stars have to be formed in the Galactic disk and be ejected shortly afterwards. Przybilla et al. 2009 developed one ejection scenario: a supernova in a close PCEB system. The much more massive primary is evolving very fast and a common envelope around both stars is formed due to unstable mass transfer. The result is a massive binary in a very short orbit. When the more massive star explodes in a core-collapse supernova, the companion may be released nearly at its orbital velocity. The atmosphere of the runaway star can be polluted with the supernova ejecta in this process. Therefore, such kind of stars are ideally suited to study nucleosynthesis in a core collapse supernova. HD 271791 is the only known runaway star, whose galactic restframe velocity (740+-150 km/s) exceeds the escape velocity of the Galaxy. Hence, it is called a hyper-runaway star. A prior investigation of this star by Przybilla et al. 2009 has shown an enhancement of the alpha-process elements. This indicates the capture of supernova ejecta. As such high space velocities are not reached by the runaway stars in classical binary supernova ejection scenarios, a very massive but compact primary, probably of Wolf-Rayet type, is required, which is expected to be formed by ejecting the envelope in a common-envelope phase. A later theoretical investigation by Gvaramadze 2009 of this system put the acceleration scenario in question, finding it unlikely to accelerate a star with this properties to such high velocities. The first quantitative analysis of the star was based on optical spectra, which allowed abundances of only a small number of elements to be determined. More chemical elements can only be investigated with the help of ultraviolet spectra. Therefore, Hubble Space Telescope observations were obtained. The goal of this project is to further investigate nucleosynthesis in a core-collapse supernova. Hence, the spectrum synthesis computations were extended to the UV to facilitate abundance determinations in this spectral region, where many chemical species produce a dense forest of spectral lines. In particular, also the lines of elements that can only be produced by rapid neutron capture (r process) are visible there. They are suggested to be synthesized in core-collapse supernovae, as high neutron fluxes are available there. Hence, it could provide evidence for the r process taking place in core-collapse supernovae, if the r-process elements were found to be enhanced. The spectrum synthesis was tested by the abundance determination of four bright B stars with different temperatures from 17500-33000 K based on the stellar parameters determined from the analysis of the optical spectra. Afterwards, we re-investigated HD 271791 based on higher quality optical spectra to determine the stellar parameters and abundances with higher accuracy. We also performed some simulations to re-investigate the velocities a star, which has the properties of HD 271791, can reach in the supernova scenario. Based on the parameters from the optical analysis, our spectrum synthesis was used to derive abundances of HD 271791 for elements until the iron group in a second step. In summary, this work establishes the basis for comprehensive abundance determinations of B stars in the UV in the future, comprising iron-group elements and many heavier chemical species. This will ultimately facilitate to investigate nucleosynthesis in a core-collapse supernova in great detail by this novel approach