Janusz Kaluzny

Cluster AgeS Experiment. CCD photometry of SX Phoenicis variables in the globular cluster M 55

We present CCD photometry of SX Phe variables in the field of the globular cluster M 55. We have discovered 27 variables, three of which are probable members of the Sagittarius dwarf galaxy. All of the SX Phe stars in M 55 lie in the blue straggler region of the cluster color-magnitude diagram. Using period ratio information we have identified the radial pulsation modes for one of the observed variables. Inspection of the period-luminosity distribution permits the probable identifications of the pulsation modes for most of the rest of the stars in the sample. We have determined the slope of the period-luminosity relation for SX Phe stars in M 55 pulsating in the fundamental mode. Using this relation and the HIPPARCOS data for SX Phe itself, we have estimated the apparent distance modulus to M 55 to be

Direct distances to nearby galaxies using detached eclipsing binaries and cepheids. vii. additional variables in the field m33a discovered with image subtraction

(m-M)_V=13.86pm0.25

Time Series Photometry of Variable Stars in the Globular Cluster NGC 6397

mag.

CCD photometry of the X-ray nova V404 Cygni after the 1989 outburst

DIRECT is a project to obtain directly the distances to two Local Group galaxies, M31 and M33, which occupy a crucial position near the bottom of the cosmological distance ladder. As the first step of the DIRECT project we have searched for detached eclipsing binaries (DEBs) and new Cepheids in the M31 and M33 galaxies with 1 m class telescopes. In this eighth paper we present a catalog of variable stars discovered in the data from the follow-up observations of DEB system D33J013337.0+303032.8 in field M33B [(α, δ) = (2348, 3057), J2000.0], collected with the Kitt Peak National Observatory 2.1 m telescope. In our search covering an area of 108 arcmin2 we have found 895 variable stars: 96 eclipsing binaries, 349 Cepheids, and 450 other periodic, possibly long-period or nonperiodic variables. Of these variables 612 are newly discovered. Their light curves were extracted using the ISIS image subtraction package. For 77% of the variables we present light curves in standard V and B magnitudes, with the remaining 23% expressed in units of differential flux. We have discovered a population of first-overtone Cepheid candidates, and for six of them we present strong arguments in favor of this interpretation. The catalog of variables, as well as their photometry (about 9.2 × 104 BV measurements) and finding charts, is available electronically via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

A VIMOS spectroscopy study of photometric variables and straggler candidates in ω Centauri

Time series BVI photometry is presented for 16 short-period variables located in the central region of the globular cluster NGC 6397. The sample includes nine newly detected variables. The light curve of cataclysmic variable CV6 shows variability with a period of 0.2356 days. We confirm an earlier reported period of 0.472 days for cataclysmic variable CV1. Phased light curves of both CVs exhibit sinelike light curves, with two minima occurring during each orbital cycle. The secondary component of CV1 has a low average density of 0.83 g cm-3, indicating that it cannot be a normal main-sequence star. Variables among the cluster blue stragglers include a likely detached eclipsing binary with orbital period of 0.787 days, three new SX Phoenicis stars (one of which has the extremely short period of 0.0215 days), and three low-amplitude variables that are possible γ Doradus variables.

CCD photometry of distant open clusters NGC 2425, Haffner 10 and Czernik 29

CCD photometry of the X-ray nova V404 Cyg, carried out 2.5, 6.5, 12.5, and 13 months after the May 1989 eruption, is presented. Despite a rather limited coverage, the data show evidence of the presence of a periodic modulation. The preliminary period is equal to 3.05 or 3.52 hours. Alternately, if V404 Cyg has a double-peaked light curve like other X-ray transients, the period is twice this value, about 6 hours. The amplitude of the light variations increased with fading brightness of the nova and was about 0.5 m in June 1990. It has been found that V404 Cyg has a close optical companion separated by 1.4 arcsec which is brighter than the nova at minimum. Based on the measurements and preoutburst photometry, it is estimated that the secondary component of the V404 Cyg system is a main-sequence F0-K2 star or a slightly evolved star if the spectral type is later than K2. This may explain why the nova differs during the outburst from other X-ray novae where the secondaries are usually late-K-type main-sequence stars.

Variable Stars in the Large Magellanic Cloud: Discovery of Extragalactic W UMa Binaries*

We report a spectroscopic study of 19 photometric variables and 55 blue, yellow and red straggler candidates in the field of omega Centauri. We confirm the cluster membership of 18 variables and 54 straggler candidates. Velocity variations are detected in 22 objects, and another 17 objects are classified as suspected of being velocity-variable. Velocity data of 11 photometric variables phase with their photometric periods, however none of these objects has a mass function indicating the presence of a massive degenerate component. Based on both photometric and spectroscopic data we find that the fraction of binaries among blue stragglers may be as high as 69 per cent.

Open Cluster LW 55 in the Large Magellanic Cloud

We present BVI photometry for the poorly known southern hemisphere open clusters NGC 2425, Haffner 10 and Czernik 29. We have calculated the density profile and established the number of stars in each cluster. The colour-magnitude diagrams of the objects show a well-defined main sequence. However, the red giant clump is present only in NGC 2425 and Haffner 10. For these two clusters we estimated the age as 2.5 ± 0.5 Gyr, assuming a metallicity of Z = 0.008. The apparent distance moduli are in the ranges 13.2 < (m - M) v < 13.6 and 14.3< (m - M) v < 14.7, while the heliocentric distances are estimated to be 2.9 < d < 3.8 kpc and 3.1 < d < 4.3 kpc for NGC 2425 and Haffner 10, respectively. The angular separation of 2°4 (150 pc at mean distance) may indicate a common origin of the two clusters.

CCD Photometry of Faint Variable Stars in the Field of the Globular Cluster M4

We observed a field in the disk of the LMC on two consecutive nights in search of rapid variable stars. We found two pulsating stars of type RRab and δ Scuti and four binary stars; among the latter we found one sdB or cataclysmic variable below the LMC blue main sequence and three very close binary systems on the main sequence. At least one of the main-sequence binaries, and possibly all three, are the first solar-type (W UMa–type) contact binaries to be detected in any extragalactic system and observed to obey the same MV = MV(log P, B - V) calibration as the Galactic systems. Given the selection effects due to small amplitudes at faint magnitudes, the frequency of such binaries in the disk of the LMC with its large spread in population ages is not inconsistent with that in the disk of our Galaxy and contrasts with the lack of binaries found in earlier observations of the much younger LMC cluster LW 55.

The Optical Gravitational Lensing Experiment. The Early Warning System: Real Time Microlensing

The field of the moderately old open cluster LW 55 in the outer parts of the Large Magellanic Cloud disk was monitored for detection of short-period variable stars over two nights. Eight variables were found; all but one (a detached eclipsing binary) are small-amplitude (ΔV ≤ 0.1 mag) pulsating stars of the δ Scuti, SX Phoenicis, or γ Doradus type. The BV color-magnitude diagram extends down to the solar-type stars at V = 24 (MV 5.2). The cluster age is estimated at 1.5 Gyr for Z = 0.004 ([Fe/H] = -0.67); the surrounding stellar field is older, with an age greater than 4 Gyr.