The magnetic white dwarf SDSS J121209.31+013627.7 exhibits a weak, narrow Halpha emission line whose radial velocity and strength are modulated on a period of ~90 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_J > 13.37. This is equivalent to an isolated L5 dwarf, with T_eff < 1700 K. Consideration of possible evolutionary histories suggests that, until ~0.6 Gyr ago, the brown dwarf orbited a ~1.5 M_sun main seqeunce star with P ~ 1 yr, a ~ 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.
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