Detection of a Variable Infrared Excess around SDSS J121209.31+013627.7

Abstract

We present near-infrared JHKs photometry 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 Ks band. 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 Ks light curve, we get a refined period for the system of 88 ± 1 minutes. We show that the observed variability in the Ks band 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.