HATnet variability survey in the high stellar density "Kepler field" with millimagnitude image subtraction photometry

The Hungarian-made Automated Telescope Network (HATnet) is an ongoing project to detect transiting extrasolar planets using small-aperture ( 11 cm diameter) robotic telescopes. In this paper, we present the results from using image subtraction photometry to reduce a crowded stellar field observed with one of the HATnet telescopes (HAT-5). This field was chosen to overlap with the planned Kepler mission. We obtained I-band light curves for 98,000 objects in a 67 square degree field of view centered at J2000 (alpha, delta) = (19(h)44(m)00:(s)0, + 37degrees32'00."0), near the Galactic plane in the constellations Cygnus and Lyra. These observations include 788 exposures of 5 minutes' length over 30 days. For the brightest stars (I similar to 8.0) we achieved a precision of 3.5 mmag, falling to 0.1 mag at the faint end ( I similar to 14). From these light curves we identify 1617 variable stars, of which 1439 are newly discovered. The fact that nearly 90% of the variables were previously undetected further demonstrates the vast number of variables yet to be discovered even among fairly bright stars in our Galaxy. We also discuss some of the most interesting cases. These include V1171 Cyg, a triple system with the inner two stars in a P = 1.462 day period eclipsing orbit and the outer star a P = 4.86 day Cepheid; HD 227269, an eccentric eclipsing system with a P = 4.86 day period that also shows P = 2.907 day pulsations; WW Cyg, a well-studied eclipsing binary; V482 Cyg, an R Coronae Borealis star; and V546 Cyg, a PV Telescopii variable. We also detect a number of small-amplitude variables, in some cases with full amplitude as low as 10 mmag.