Program: GN-2009A-Q-19

Binary evolution theory has long predicted a population of white dwarfs accreting from sub-stellar donors, but the secure identification of such a system has proved impossible until recently. By studying eclipsing systems, and using a parametric model of the eclipse lightcurves we have discovered four systems with substellar donor stars, providing an important and long overdue validation of binary evolution theory. Located in eclipsing binaries, these systems are of great use in probing the mass-radius relationship in the substellar regime. Unfortunately, the measured masses and radii of our objects are inconsistent with state-of-the-art stellar models. We find the substellar donor stars are 10-15% larger than expected. Possible explanations include flaws in the models (missing opacity, or the effects of magnetism), or the consequences of binarity, for example irradiation from the white dwarf. We propose a spectroscopic detection of the donor star in one of these systems to distinguish between these possibilities.