CCD Photometry of Variable Stars in the Field of the Globular Cluster NGC 6397

The $13.6\times 13.6$ arcmin$^{2}$ field covering the central part of the globular cluster NGC~6397 was surveyed in a search for short-period variable stars. We obtained light curves for 6 close binaries, 2 SX~Phe stars and 1 RR~Lyrae variable, 7 of which are new discoveries. Both identified SX~Phe variables are likely members of the cluster. One of the identified eclipsing binaries is most probably a field star while for the remaining 5 binaries our data do not preclude cluster membership. A $V$ vs. $B-V$ color-magnitude diagram extending from the tip of the red giant branch to below the main-sequence turnoff is presented.Comment: 14 pages, 6 figures, uuencoded postscript, submitted to A&

Long-term variability survey of the old open cluster NGC 6791

We present the results of a long-term variability survey of the old open cluster NGC 6791. The BVI observations, collected over a time span of 6 years, were analyzed using the ISIS image subtraction package. The main target of our observations were two cataclysmic variables B7 and B8. We have identified possible cycle lenghts of about 25 and 18 days for B7 and B8, respectively. We tentatively classify B7 as a VY Scl type nova-like variable or a Z Cam type dwarf nova. B8 is most likely an SS Cygni type dwarf nova. We have also extracted the light curves of 42 other previously reported variable stars and discovered seven new ones. The new variables show long-period or non-periodic variability. The long baseline of our observations has also allowed us to derive more precise periods for the variables, especially for the short period eclipsing binaries.Comment: 13 pages LaTeX, including 8 PostScript figures and 4 tables. To appear in June 2003 issue of The Astronomical Journa

UBVI CCD Photometry of the Old Open Cluster Berkeley 17

Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17. Color-magnitude diagrams (CMDs) are constructed and reach well past the main-sequence turn-off for both clusters. Cluster ages are determined by means of isochrone fitting to the cluster CMDs. These fits are constrained to agree with spectroscopic metallicity and reddening estimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188, and 10.0+/- 1.0 Gyr for Berkeley 17, where the errors refer to uncertainties in the relative age determinations. These ages are compared to the ages of relatively metal-rich inner halo/thick disk globular clusters and other old open clusters. Berkeley 17 and NGC 6791 are the oldest open clusters with an age of 10 Gyr. They are 2 Gyr younger than the thick disk globular clusters. These results confirm the status of Berkeley 17 as one of the oldest known open cluster in the Milky Way, and its age provides a lower limit to the age of the Galactic disk.Comment: to appear in AJ; 28 pages, 9 figure

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 (m-M)_V=13.86 +- 0.25 mag.Comment: A&A accepted, 11 figure

CCD Photometry of Variable Stars in the Globular Cluster NGC 288

The central part of the globular cluster NGC 288 was surveyed in a search for short-period variable stars. We obtained the V-band light curves for 4 SX Phe stars and 2 RR Lyrae variables, five of which are new discoveries. All SX Phe stars belong to blue stragglers. We present a color-magnitude diagram including stars from the tip of red giant branch to the level of the main-sequence turnoff. Three likely hot subdwarfs were located on the extension of the blue horizontal branch of the cluster.Comment: 12 pages including 6 figures, uuencoded postscript file, submitted to A&