The Unusual Cataclysmic Binary Star RBS 0490 and the Space Density of Cataclysmic Variables

RBS (ROSAT Bright Source) 0490 is a cataclysmic variable star (CV) with unusually strong emission lines. The strength of the emission lines has led to a suggestion that the object is intrinsically faint and correspondingly nearby (∼33 pc), which, if true, would strongly affect estimates of the CV space density. Here we report astrometry, filter photometry, and time‐series spectroscopy of this object. The astrometry gives an absolute parallax πabs = 4.5 ± 1.5 mas and a relative proper motion of 102 mas yr−1. A Bayesian procedure gives a very uncertain distance estimate of d∼300 pc, and the small parallax alone implies d>133 pc (at 2 standard deviations). The mean V magnitude is 17.4, which implies MV = 10.9 - 5log [d/(200 pc)], neglecting extinction. At 200 pc, the space velocity would be over 90 km s−1 with respect to the local standard of rest. The time‐series spectroscopy shows a possible emission‐line radial velocity period near 46 minutes. This would be unusually short for an orbital period, and it may represent some other clock in the system.