The first sub-70 min non-interacting WD–BD system: EPIC212235321

Abstract

We present the discovery of the shortest-period, non-interacting, white\ndwarf-brown dwarf post-common-envelope binary known. The K2 light curve shows\nthe system, EPIC 21223532 has a period of 68.2 min and is not eclipsing, but\ndoes show a large reflection effect due to the irradiation of the brown dwarf\nby the white dwarf primary. Spectra show hydrogen, magnesium and calcium\nemission features from the brown dwarf's irradiated hemisphere, and the mass\nindicates the spectral type is likely to be L3. Despite having a period\nsubstantially lower than the cataclysmic variable period minimum, this system\nis likely a pre-cataclysmic binary, recently emerged from the common-envelope.\nThese systems are rare, but provide limits on the lowest mass object that can\nsurvive common envelope evolution, and information about the evolution of white\ndwarf progenitors, and post-common envelope evolution.\n