Discovery of a cataclysmic variable with a sub-stellar companion

We find that the ROSAT source 1RXS J105010.3-140431 is a cataclysmic variab le with orbital period of 88.6 min and a spectrum closely resembling WZ Sge . In particular, emission lines are flanked by Stark-broadened absorption w ings probably originating in the photosphere of a compact object. The Balme r absorption lines can be modeled by the spectrum of a DA white dwarf with 13 000 < T-eff < 24 000 K. The strong absorption lines allowed us to obtain direct radial velocities of the white dwarf using the cross-correlation te chnique. We find an extremely low white dwarf radial velocity half amplitud e, K-wd = 4 +/- 1 km s(-1). This is consistent with the upper limit obtaine d from the H alpha emission line wing K < 20 km s(-1). The corresponding ma ss function is incompatible with a main sequence secondary, but is compatib le with a post orbital period minimum cataclysmic variable with a brown dwa rf-like secondary. The formal solution gives a secondary mass of 10-20 Jovi an masses. Doppler maps for the emission lines and the hypothesis of black- body emission indicate a steady state (T <similar to> tau (-3/4)) accretion disk mainly emitting in H alpha and an optically thicker hotspot with a st rong contribution to the higher order Balmer lines and He I 5875. As in oth er long cycle length dwarf novae, evidence for inner disk removal is found from the analysis of the emission lines.