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

Search for Variable Stars in the Globular Cluster M3

We describe here results of a photometric time-sequence survey of the globular cluster M3 (NGC 5272), in a search for contact and detached eclipsing binary stars. We have discovered only one likely eclipsing binary and one SX Phe type star in spite of monitoring 4077 stars with $V<20.0$ and observing 25 blue stragglers. The newly identified SX Phe star, V237, shows a light curve with a variable amplitude. Variable V238 shows variability either with a period of 0.49 d or with a period of 0.25 d. On the cluster colour-magnitude diagram, the variable occupies a position a few hundredths of magnitude to the blue of the base of the red giant branch. V238 is a likely descendent of a binary blue straggler. As a side result we obtained high quality data for 42 of the previously known RR Lyrae variables, including 33 of Bailey type ab, 7 type c and 2 double-mode pulsators. We used equations that relate the physical properties of RRc stars to their pulsation periods and Fourier parameters to derive masses, luminosities, temperatures and helium parameters for five of the RRc stars. One of the RRd stars (V79) has switched modes. In previous studies, it was classified as RRab, but our observations show that it is an RRd star with the first overtone mode dominating. This indicates blueward evolution on the horizontal branch.Comment: 21 pages including 14 figures, Latex, requires mn.sty, psfig.sty. Submitted, MNRA

Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low Mass White Dwarfs

We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ 0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS spectra, suggests that most of these stars are undermassive, = 0.43 +/- 0.06 Msun, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791's evolved stars must have lost enough mass on the red giant branch to avoid the flash, and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen core stars and therefore the corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.Comment: 15 pages, 9 figures, 2 tables. Accepted for publication in Astrophys. J. Very minor changes from first versio