  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.
