2006ApJ...647L.147D

We present near-infrared JHK_sphotometry and light curves of the candidate magnetic white dwarf+brown dwarf binary SDSS J121209.31+013627.7 and report on the detection of near-infrared excess and variability in the K_sband. The observed near-infrared excess can be explained by the presence of an L7 brown dwarf and an extra emission source. The J and H light curves appear flat, which rules out eclipses deeper than 0.2 mag and the presence of an accretion hot spot on the white dwarf. From the variable K_s light curve, we get a refined period for the system of 88±1 minutes. We show that the observed variability in the K_sband can be explained by cyclotron emission, which can be modeled by a small spot on the surface of the white dwarf. SDSS J121209.31+013627.7 exhibits similarities to the ultra-short-period Polar EF Eridani; however, the lack of evidence for Roche lobe overflow accretion suggests that it may be a pre-Polar.