Photometric survey of stellar clusters in the outer part of M33. II. Analysis of HST/ACS images

We have used deep ACS/WFC images of M33 to check nature of extended objects detected by the ground based survey of Zloczewski et al. (2008). A total of 24 candidates turned out to be genuine compact stellar clusters. In addition we detected 91 new clusters. Equatorial coordinates, integrated magnitudes and angular sizes are listed for all 115 objects. Fourty-two clusters have sufficiently red colors to be candidates for old globulars. For four clusters we extracted resolved stellar photometry. Object 33-3-018 located in the outer disk of M33 turned out to be a young cluster with an age estimated at 200-350 Myr. Cluster ZK-90 has an age of 3-5 Gyr. The remaining two clusters have intermediate ages ranging from one to a few Gyr.Comment: 22 pages, 9 figures, accepted to Acta Astronomica on 22.03.2009; minor changes for ZK-90 and Figure 1

The 2003 and 2005 superhumps in V1113 Cygni

We report CCD photometry of the cataclysmic variable V1113 Cygni. During two campaigns, lasting from May to August 2003 and from March to June 2005, we recorded two superoutburst. In the obtained light curves we detected clear superhumps with a mean period 0.07891(3) days (113.63(4) min). That fact confirms that the star is a member of SU UMa class of dwarf novae. During the first observed superoutburst the superhump period was decreasing with an enormous rate of $\dot P = -4.5(8)\times 10^{-4}$ which is one of the highest values ever observed in SU UMa systems.Comment: 9 pages, 5 figures, accepted for publication in Acta Astronomic

An Unusual Eclipsing Blue Straggler V8-NGC 6752

We report the analysis of a binary blue straggler in NGC 6752 with a short orbital period of 0.315 d and a W UMA-type light curve. We use photometric data spanning 13 years to place limits on the mass ratio (0.15<q<0.35), luminosity ratio (L1/L2 about 4.0) and the ratio of the radii of the components (r1/r2 about 2.0). The effective temperatures of the components are nearly identical, and the system is detached or semi-detached (in the latter case the component filling its Roche lobe is the secondary). Such a configuration is unusual given the shortness of the orbital period, and it must have resulted from substantial mass exchange. We suggest that some secondaries of W UMa-type stars, normally regarded as main sequence objects which fill their Roche lobes to different degrees, in fact may be shell-burning cores of originally more massive components.Comment: 13 pages, 5 figures, submitted to Acta Astronomic

The Cluster AgeS Experiment (CASE). Variable Stars in the Globular Cluster M55

We report time-series photometry for 55 variable stars located in the central part of the globular cluster M55. The sample includes 28 newly identified objects of which 13 are eclipsing binaries. Three of these are detached systems located in the turn-off region on the cluster color-magnitude diagram. Two of them are proper motion (PM) members of M55 and are excellent candidates for a detailed follow-up study aimed at a determination of the cluster age and distance. Other detached binaries are located along the unevolved part of the cluster main sequence. Most of the variables are cluster blue straggler stars. This group includes 35 SX Phe stars, two contact binaries, and one semi-detached binary. V60 is a low mass, short period algol with the less massive and cooler component filling its Roche lobe. The more massive component is an SX Phe variable. The orbital period of V60 increases at a rate of dP/P=3.0E-9. In addition to numerous variable blue stragglers we also report the detection of two red stragglers showing periodic variability. Both of these are PM members of M55. We note and discuss the observed paucity of contact binaries among unevolved main sequence stars in M55 and NGC 6752. This apparent paucity supports an evolution model in which the formation of contact binaries is triggered by stellar evolution at the main-sequence turn off.Comment: 22 pages, 12 figures, published in Acta Astronomic

The Clusters AgeS Experiment (CASE): Variable Stars in the Globular Cluster 47 Tucanae

Based on over 5400 BV images of 47 Tuc collected between 1998 and 2010 we obtained light curves of 65 variables, 21 of which are newly detected objects. New variables are located mostly just outside of the core in a region poorly studied by earlier surveys of the cluster. Among them there are four detached eclipsing binaries and five likely optical counterparts of X-ray sources. Two detached systems are promising targets for follow-up observations. We briefly discuss properties of the most interesting new variables.Comment: Accepted by Acta Astronomic

Curious Variables Experiment (CURVE). CCD Photometry of Dwarf Nova V660 Herculis

We report extensive photometry of the dwarf nova V660 Herculis. During our campaign, lasting from August 2003 to November 2004, we recorded one bright eruption which turned out to be a superoutburst lasting about 15 days and having amplitude of ~4.5 mag. Clear superhumps with a mean period of Psh = 0.080924(18) days were present during all nights of the superoutburst. The period of the superhumps was not stable and in the interval covered by our observations it decreased with a rate of -4.0(1.4) * 10^{-5}. Basing on our data and the known orbital period of the binary (Thorstensen and Fenton 2003) we calculate the period excess of 3.4 +/- 0.1%, which is typical for an SU UMa star at this orbital period. This value indicates that the mass ratio of the system is q=0.154.Comment: 11 pages, 7 figures, Acta Astronomica, in prin

Curious Variables Experiment (CURVE). Superhump Period Change Pattern in KS UMa and Other Dwarf Novae

We report extensive photometry of the dwarf nova KS UMa throughout its 2003 superoutburst till quiescence. During the superoutburst the star displayed clear superhumps with a mean period of Psh = 0.070092(23) days. In the middle stage of superoutburst the period was increasing with a rate of $\dot P/P = (21\pm12)\times 10^{-5}$ and later was decreasing with a rate of $\dot P/P = -(21\pm8)\times 10^{-5}$. At the end of superoutburst and during first dozen days of quiescence the star was showing late superhumps with a mean period of Plate = 0.06926(2) days. This phenomenon was observed even 30 days after beginning of the superoutburst. In quiescence the star shows quasi-periodic modulations with amplitude reaching 0.5 mag. The most common structure observed during this stage was sinusoidal wave characterized by a period of about 0.1 days. Comparing KS UMa to other SU UMa stars we conclude that this group of dwarf novae shows decreasing superhump periods at the beginning and the end of superoutburst but increasing period in the middle phase.Comment: submitted to Acta Astronomica, 21 pages, 13 figure

Curious Variables Experiment (CURVE). RZ LMi - the most active SU UMa star

We report extensive photometry of the frequently outbursting dwarf nova RZ Leo Minoris. During two seasons of observations we detected 12 superoutbursts and 7 normal outbursts. The V magnitude of the star varied in range from 16.5 to 13.9 mag. The superoutbursts occur quite regularly flashing every 19.07(4) days and lasting slightly over 10 days. The average interval between two successive normal outbursts is 4.027(3) days. The mean superhump period observed during the superoutbursts is 0.059396(4) days (85.530(6) min). The period of the superhumps was constant except for one superoutburst when it increased with a rate of $\dot P/P_{\rm sh} = 7.6(1.9)\cdot 10^{-5}$. Our observations indicate that RZ LMi goes into long intervals of showing permanent superhumps which are observed both in superoutbursts and quiescence. This may indicate that decoupling of thermal and tidal instabilities play important role in ER UMa systems. No periodic light variations which can be connected with orbital period of the binary were seen, thus the mass ratio and evolutionary status of RZ LMi are still unknown.Comment: 20 pages, 15 figures, accepted for publication in Acta Astronomic

A proper motion study of the globular clusters M4, M12, M22, NGC 3201, NGC 6362 and NGC 6752

We derive relative proper motions of stars in the fields of globular clusters M4, M12, M22, NGC 3201, NGC 6362 and NGC 6752 based on a uniform data set collected between 1997 and 2008. We assign a membership class for each star with a measured proper motion, and show that these membership classes can be successfully used to eliminate field stars from color-magnitude diagrams of the clusters. They also allow for the efficient selection of rare objects such as blue/yellow/red stragglers and stars from the asymptotic giant branch. Tables with proper motions and photometry of over 87000 stars are made publicly available via the Internet.Comment: 9 pages, 18 figure

Curious Variables Experiment (CURVE). TT Bootis - superhump period change pattern confirmed

We report extensive multi-station photometry of TT Boo during its June 2004 superoutburst. The amplitude of the superoutburst was about 5.5 mag and its length over 22 days. The star showed a small re-brightening starting around the 9th day of the superoutburst. During entire bright state we observed clear superhumps with amplitudes from 0.07 to 0.26 mag and a mean period of Psh = 0.0779589(47) days (112.261 +/- 0.007 min). The period was not constant but decreased at the beginning and the end of superoutburst yet increased in the middle phase. We argue that the complicated shape of the O-C diagram is caused by real period changes rather than by phase shifts. Combining the data from two superoutbursts from 1989 and 2004 allowed us to trace the birth of the late superhumps and we conclude that it is a rather quick process lasting about one day.Comment: 14 pages, 6 figures, submitted to Acta Astronomic