The Solar-Type Contact Binary BX Pegasi Revisited

We present the results of new CCD photometry for the contact binary BX Peg, made during three successive months beginning on September 2008. As do historical light curves, our observations display an O'Connell effect and the November data by themselves indicate clear evidence for very short-time brightness disturbance. For these variations, model spots are applied separately to the two data set of Group I (Sep.--Oct.) and Group II (Nov.). The former is described by a single cool spot on the secondary photosphere and the latter by a two-spot model with a cool spot on the cool star and a hot one on either star. These are generalized manifestations of the magnetic activity of the binary system. Twenty light-curve timings calculated from Wilson-Devinney code were used for a period study, together with all other minimum epochs. The complex period changes of BX Peg can be sorted into a secular period decrease caused dominantly by angular momentum loss due to magnetic stellar wind braking, a light-travel-time (LTT) effect due to the orbit of a low-mass third companion, and a previously unknown short-term oscillation. This last period modulation could be produced either by a second LTT orbit with a period of about 16 yr due to the existence of a fourth body or by the effect of magnetic activity with a cycle length of about 12 yr.Comment: 23 pages, including 6 figures and 8 tables, accepted for publication in PAS

Physical Properties of the Transiting Planetary System TrES-3

We present four new transits of the planetary system TrES-3 observed between 2009 May and 2010 June. Among these, the third transit by itself indicates possible evidence for brightness disturbance, which might be the result of the planet blocking a cool starspot on the stellar surface. A total of 109 transit times, including our measurements, were used to determine the improved ephemeris with a transit epoch of 2454185.910944$\pm$0.000072 HJED and an orbital period of 1.30618700$\pm$0.00000015 d. We analyzed the transit light curves using the JKTEBOP code and adopting the quadratic limb-darkening law. In order to derive the physical properties of the TrES-3 system, the transit parameters are combined with the empirical relations from eclipsing binary stars and stellar evolutionary models. The stellar mass and radius obtained from a calibration using $T_A$, log $\rho_{\rm A}$ and [Fe/H] are consistent with those from the isochrone analysis. We found that the exoplanet TrES-3b has a mass of 1.93$\pm$0.07 M$_{\rm Jup}$, a radius of 1.30$\pm$0.04 R$_{\rm Jup}$, a surface gravity of log $g_{\rm b}$=3.45$\pm$0.02, a density of 0.82$\pm$0.06 $\rho_{\rm Jup}$, and an equilibrium temperature of 1641$\pm$23 K. The results are in good agreement with theoretical models for gas giant planets.Comment: 15 pages, including 4 figures and 4 tables, accepted for publication in PAS

The sub-Saturn Mass Transiting Planet HAT-P-12b

We present new photometric data of the transiting planet HAT-P-12b observed in 2011. Our three transit curves are modelled using the JKTEBOP code and adopting the quadratic limb-darkening law. Including our measurements, 18 transit times spanning about 4.2 yr were used to determine the improved ephemeris with a transit epoch of 2,454,187.85560$\pm$0.00011 BJD and an orbital period of 3.21305961$\pm$0.00000035 d. The physical properties of the star-planet system are computed using empirical calibrations from eclipsing binary stars and stellar evolutionary models, combined with both our transit parameters and previously-known spectroscopic results. We found that the absolute dimensions of the host star are $M_{\rm A}$=0.73$\pm$0.02 M$_\odot$, $R_{\rm A}$=0.70$\pm$0.01 R$_\odot$, log $g_{\rm A}$=4.61$\pm$0.02, $\rho_{\rm A}$=2.10$\pm$0.09 $\rho_{\odot}$, and $L_{\rm A}$=0.21$\pm$0.01 L$_\odot$. The planetary companion has $M_{\rm b}$=0.21$\pm$0.01 M$_{\rm Jup}$, $R_{\rm b}$=0.94$\pm$0.01 R$_{\rm Jup}$, log $g_{\rm b}$=2.77$\pm$0.02, $\rho_{\rm b}$=0.24$\pm$0.01 $\rho_{\rm Jup}$, and $T_{\rm eq}$=960$\pm$14 K. Our results agree well with standard models of irradiated gas giants with a core mass of 11.3 M$_{\oplus}$.Comment: 17 pages, including 5 figures and 5 tables, accepted for publication in A

The Period Variation of and a Spot Model for the Eclipsing Binary AR Bootis

New CCD photometric observations of the eclipsing system AR Boo were obtained from February 2006 to April 2008. The star's photometric properties are derived from detailed studies of the period variability and of all available light curves. We find that over about 56 years the orbital period of the system has varied due to a combination of an upward parabola and a sinusoid rather than in a monotonic fashion. Mass transfer from the less massive primary to the more massive secondary component is likely responsible for at least a significant part of the secular period change. The cyclical variation with a period of 7.57 yrs and a semi-amplitude of 0.0015 d can be produced either by a light-travel-time effect due to an unseen companion with a scaled mass of $M_3 \sin i_3$=0.081 $M_\odot$ or by a magnetic period modulation in the secondary star. Historical light curves of AR Boo, as well as our own, display season-to-season light variability, which are best modeled by including both a cool spot and a hot one on the secondary star. We think that the spots express magnetic dynamo-related activity and offer limited support for preferring the magnetic interpretation of the 7.57-year cycle over the third-body understanding. Our solutions confirm that AR Boo belongs to the W-subtype contact binary class, consisting of a hotter, less massive primary star with a spectral type of G9 and a companion of spectral type K1.Comment: 30 pages, including 6 figures and 9 tables, accepted for publication in A