Discovery of a Magnetic White Dwarf/Probable Brown Dwarf Short-Period Binary

The magnetic white dwarf SDSS J121209.31+013627.7 exhibits a weak, narrow Ha emission line whose radial velocity and strength are modulated on a period of 690 minutes. Though indicative of irradiation on a nearby companion, no cool continuum component is evident in the optical spectrum, and IR photometry limits the absolute magnitude of the companion to M sub(J)> 13.37. This is equivalent to an isolated L5 dwarf, with T sub(eff) < 1700 K. Consideration of possible evolutionary histories suggests that, until 60.6 Gyr ago, the brown dwarf orbited a 61.5 M sub( )main-sequence star with P 6 1 yr and a6 1 AU, thus resembling many of the gaseous superplanets being found in extrasolar planet searches. Common-envelope evolution when the massive star left the main sequence reduced the period to only a few hours, and ensuing angular momentum loss has further degraded the orbit. The binary is ripe for additional observations aimed at better studying brown dwarfs and the effects of irradiation on their structure.