WD 1202-024: the shortest-period pre-cataclysmic variable

Among the 28,000 targeted stars in K2 Field 10 is the white dwarf WD 1202-024 (EPIC 201283111), first noted in the SDSS survey (SDSS 120515.80-024222.7). We have found that this hot white dwarf (Teff = 22,640 K) is in a very close orbit (P = 71 min) with a star of near brown-dwarf mass ~ 0.061 Msun. This period is very close to, or somewhat below, the minimum orbital period of cataclysmic variables with H-rich donor stars. However, we find no evidence that this binary is currently, or ever was, transferring mass from the low-mass companion to the white dwarf. We therefore tentatively conclude that this system is still in the pre-cataclysmic variable phase, having emerged from a common envelope some 50 +/- 20 Myr ago. Because of the 29-minute integration time of K2, we use follow-up ground-based photometry to better evaluate the eclipsing light curve. We also utilize the original SDSS spectra, in approximately 15-min segments, to estimate the radial velocity of the white dwarf in its orbit. An analysis of the light curve, with supplementary constraints, leads to the following system parameters: Mwd = 0.415 +/- 0.028 Msun, Rwd = 0.021 +/- 0.001 Rsun, Mcom = 0.061 +\- 0.010 Msun, and Rcom = 0.088 +\- 0.005 Rsun where the subscripts 'wd' and 'com' refer to the white dwarf and low-mass companion respectively. If our interpretation of this system as a pre-CV is correct, it has the shortest period of any such system yet found and should become a compact CV in less than 250 Myr.