A newly - discovered cataclysmic binary near the ROSAT galactic plane source RX J 1910 . 8 + 2856

We report the discovery of a new cataclysmic binary star from the ROSAT Galactic Plane Survey. The star has V >∼ 18.5 at minimum light and we have measured it as bright as V = 15.7. Spectroscopy shows broad hydrogen and helium emission lines; HeII λ4686 is not detected. Hα radial velocities yield an orbital period of 0.1429 ± 0.0004 d (3.430 ± 0.010 h). Timeseries photometry covering one orbital cycle near maximum light shows irregular variability, but no evidence of an eclipse. The system appears to be a new dwarf nova of the U Gem or Z Cam type, and is unlikely to be a magnetic system. Its period is unusually short for a U Gem star. Curiously, the new cataclysmic binary is 83′′ from the original ROSAT PSPC survey position, which puts it outside the PSPC positional uncertainty. Follow-up observations with the HRI reveal that the PSPC source is the superposition of two sources, and the cataclysmic is much the weaker of the two in X-rays. Its discovery in this survey is therefore essentially accidental. A V = 19.4 mag star is located 1.2′′ from the stronger source’s HRI position.