DISCOVERY OF A BRIGHT, EXTREMELY LOW MASS WHITE DWARF IN A CLOSE DOUBLE DEGENERATE SYSTEM

We report the discovery of a bright (V similar to 13.7), extremely low mass white dwarf in a close double degenerate system. We originally selected GALEX J171708.5+675712 for spectroscopic follow-up among a group of white dwarf candidates in an ultraviolet-optical reduced proper-motion diagram. The new white dwarf has a mass of 0.18 M-circle dot and is the primary component of a close double degenerate system (P = 0.246137 days, K-1 = 288 km s(-1)) comprising a fainter white dwarf secondary with M-2 approximate to 0.9 M-circle dot. Light curves phased with the orbital ephemeris show evidence of relativistic beaming and weaker ellipsoidal variations. The light curves also reveal secondary eclipses (depth approximate to 8 mmag) while the primary eclipses appear partially compensated by the secondary gravitational deflection and are below detection limits. Photospheric abundance measurements show a nearly solar composition of Si, Ca, and Fe (0.1-1 circle dot), while the normal kinematics suggest a relatively recent formation history. Close binary evolutionary scenarios suggest that extremely low mass white dwarfs form via a common-envelope phase and possible Roche lobe overflow.