SDSS J 150722 . 30 + 523039 . 8 : a CV formed directly from a detached white dwarf / brown dwarf binary ?

We present high-speed, three-colour photometry of the eclipsing cataclysmic variable SDSS J150722.30+523039.8 (hereafter SDSS J1507). This system has an orbital period of 66.61 minutes, placing it below the observed “period minimum” for cataclysmic variables. We determine the system parameters via a parameterised model of the eclipse fitted to the observed lightcurve by χ minimisation. We obtain a mass ratio of q = 0.0623 ± 0.0007 and an orbital inclination i = 83◦.63 ± 0◦.05. The primary mass is Mw = 0.90± 0.01M⊙. The secondary mass and radius are found to be Mr = 0.056± 0.001M⊙ and Rr = 0.096± 0.001R⊙ respectively. We find a distance to the system of 160 ± 10 pc. The secondary star in SDSS J1507 has a mass substantially below the hydrogen burning limit, making it the second confirmed sub-stellar donor in a cataclysmic variable. The very short orbital period of SDSS J1507 is readily explained if the secondary star is nuclearly evolved, or if SDSS J1507 formed directly from a detached white dwarf/brown dwarf binary. Given the lack of any visible contribution from the secondary star, the very low secondary mass and the low He I λ6678/Hα emission line ratio, we argue that SDSS J1507 probably formed directly from a detached white dwarf/brown dwarf binary. If confirmed, SDSS J1507 will be the first such system identified. The implications for binary star evolution, the brown-dwarf desert and the common envelope phase are discussed.