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We report the discovery of a bright (V ~ 13.7), extremely low-mass whitedwarf in a close double degenerate system. We originally selected GALEXJ171708.5+675712 for spectroscopic follow-up among a group of white dwarfcandidates in an ultraviolet-optical reduced proper-motion diagram. The newwhite dwarf has a mass of 0.18 M_solar and is the primary component of a closedouble degenerate system (P=0.246137 d, K_1 = 288 km/s) comprising a fainterwhite dwarf secondary with M_2 ~ 0.9 M_solar. Light curves phased with theorbital ephemeris show evidence of relativistic beaming and weaker ellipsoidalvariations. The light curves also reveal secondary eclipses (depth ~ 8 mmag)while the primary eclipses appear partially compensated by the secondarygravitational deflection and are below detection limits. Photospheric abundancemeasurements show a nearly solar composition of Si, Ca, and Fe (0.1-1 solar),while the normal kinematics suggest a relatively recent formation history.Close binary evolutionary scenarios suggest that extremely low mass whitedwarfs form via a common-envelope phase and possible Roche-lobe overflow.

Discovery of a Bright, Extremely Low-Mass White Dwarf in a Close Double Degenerate System