We report that Kepler Object of Interest 256 (KOI-256) is a mutually eclipsing post-common envelope binary (ePCEB), consisting of a cool white dwarf (M = 0.592 +/- 0.089 MSun, R = 0.01345 +/- 0.00091 RSun, Teff = 7100 +/- 700 K) and an active M3 dwarf (M = 0.51 +/- 0.16 MSun, R = 0.540 +/- 0.014 RSun, Teff = 3450 +/- 50 K) with an orbital period of 1.37865 +/- 0.00001 days. KOI-256 is listed as hosting a transiting planet-candidate by Borucki et al. and Batalha et al.; here we report that the planet-candidate transit signal is in fact the occultation of a white dwarf as it passes behind the M dwarf. We combine publicly-available long- and short-cadence Kepler light curves with ground-based measurements to robustly determine the system parameters. The occultation events are readily apparent in the Kepler light curve, as is spin-orbit synchronization of the M dwarf, and we detect the transit of the white dwarf in front of the M dwarf halfway between the occultation events. The size of the white dwarf with respect to the Einstein ring during transit (REin = 0.00473 +/- 0.00055 RSun) causes the transit depth to be shallower than expected from pure geometry due to gravitational lensing. KOI-256 is an old, long-period ePCEB and serves as a benchmark object for studying the evolution of binary star systems as well as white dwarfs themselves, thanks largely to the availability of near-continuous, ultra-precise Kepler photometry.
Cite
@article{arxiv.1304.1165,
title = {Characterizing the Cool KOIs. V. KOI-256: A Mutually Eclipsing Post-Common Envelope Binary},
author = {Philip S. Muirhead and Andrew Vanderburg and Avi Shporer and Juliette Becker and Jonathan J. Swift and James P. Lloyd and Jim Fuller and Ming Zhao and Sasha Hinkley and J. Sebastian Pineda and Michael Bottom and Andrew W. Howard and Kaspar von Braun and Tabetha S. Boyajian and Nicholas Law and Christoph Baranec and Reed Riddle and A. N. Ramaprakash and Shriharsh P. Tendulkar and Khanh Bui and Mahesh Burse and Pravin Chordia and Hillol Das and Richard Dekany and Sujit Punnadi and John Asher Johnson},
journal= {arXiv preprint arXiv:1304.1165},
year = {2013}
}