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

The DIRECT project: Catalogs of stellar objects in nearby galaxies. II. Eastern arm and NGC 206 in M31

DIRECT is a project to directly obtain the distances to two important galaxies in the cosmological distance ladder, M31 and M33, using detached eclipsing binaries and Cepheids. As part of our search for these variables, we have obtained photometry and positions for thousands of stellar objects within the monitored fields, covering an area of 557.8 arcmin^2. In this research note we present the equatorial coordinates and BVI photometry for 26712 stars in the M31 galaxy, along the eastern arm and in the vicinity of the star forming region NGC206.Comment: 2 LaTeX pages, 2 Postscript figures, submitted to Astronomy & Astrophysic

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. VIII. Additional Variables in the Field M33B Discovered with Image Subtraction

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 1m-class telescopes. In this eighth paper we present a catalog of variable stars discovered in the data from the followup observations of DEB system D33J013337.0+303032.8 in field M33B [(RA,Dec)= (23.48, 30.57), J2000.0], collected with the Kitt Peak National Observatory 2.1m telescope. In our search covering an area of 108 sq. arcmin. we have found 895 variable stars: 96 eclipsing binaries, 349 Cepheids, and 450 other periodic, possible long period or non-periodic 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*10^4 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.Comment: 21 pages of text, 27 pages of tables, 14 figures; version with full resolution figures available through ftp at ftp://cfa-ftp.harvard.edu/pub/kstanek/DIRECT/papers/M33B2

The eclipsing massive X-ray binary M33 X-7: New X-ray observations and optical identification

The eclipsing X-ray binary M33 X-7 was in the field of view during several observations of our XMM-Newton M33 survey and in the archival Chanfdra observation 1730 which cover a large part of the 3.45 d orbital period. We detect emission of M33 X-7 during eclipse and a soft X-ray spectrum of the source out of eclipse that can best be described by bremsstrahlung or disk blackbody models. No significant regular pulsations of the source in the range 0.25-1000 s were found. The average source luminosity out of eclipse is 5E37 erg/s (0.5-4.5 keV). In a special analysis of DIRECT observations we identify as optical counterpart a B0I to O7I star of 18.89 mag in V which shows the ellipsoidal heating light curve of a high mass X-ray binary with the M33 X-7 binary period. The location of the X-ray eclipse and the optical minima allow us to determine an improved binary period and ephemeris of mid-eclipse as HJD (2451760.61+-0.09)+- N * (3.45376+-0.00021). The mass of the compact object derived from orbital parameters and the optical companion mass, the lack of pulsations, and the X-ray spectrum of M33 X-7 may indicate that the compact object in the system is a black hole. M33 X-7 would be the first detected eclipsing high mass black hole X-ray binary.Comment: 9 pages including 6 figures, A&A accepte

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. V. Variables in the Field M31F

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33 -- using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, DEBs provide us with the potential to determine these distances with an accuracy better than 5%. The extensive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries, the distance estimates will be free of any intermediate steps. As a first step in the DIRECT project, between September 1996 and October 1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper, fifth in the series, we present the catalog of variable stars found in the field M31F [(\alpha,\delta)= (10.\arcdeg10, 40.\arcdeg72), J2000.0]. We have found 64 variable stars: 4 eclipsing binaries, 52 Cepheids and 8 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, is available via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.Comment: submitted to the Astronomical Journal, 31 pages, 18 figures; paper and data available at ftp://cfa-ftp.harvard.edu/pub/kstanek/DIRECT/ and through WWW at http://cfa-www.harvard.edu/~kstanek/DIRECT