Post common envelope binaries from the SDSS VI. SDSS J120615.73+510047.0 : a new low accretion rate magnetic binary

Abstract

We report the discovery of the ninth pre-polar consisting of a late-type ZAMS secondary and a magnetic white dwarf. The white dwarf accretes at an extreme low rate, \dot{M} \sim 10^ $M_\odot$ yr-1, from the wind of the companion donor star. The source was found in our systematic search for WD/MS binaries within SDSS/SEGUE. Based on seven Sloan spectra we estimate a binary period of ~200, 230, or 270 min. The UV to IR spectral energy distribution was decomposed into a dM3-dM4 ZAMS secondary and a cool white dwarf, ~9000 K, which consistently imply a distance between 360 and 420 pc. The optical spectrum displays one pronounced cyclotron hump, likely originating from a low-temperature plasma, ~1 keV, in a field of 108 MG. We comment on the evolutionary link between polars and pre-polars