A precision study of two eclipsing white dwarf plus M dwarf binaries

Abstract

We use a combination of X-shooter spectroscopy, ULTRACAM high-speed photometry and SOFI near-infrared photometry to measure the masses and radii of both components of the eclipsing post common envelope binaries SDSSJ121258.25-012310.1 and GKVir. For both systems, we measure the gravitational redshift of the white dwarf (WD) and combine it with light-curve model fits to determine the inclinations, masses and radii. For SDSSJ1212-0123, we find an inclination of i= 857 ± 05, masses of M WD= 0.439 ± 0.002M ⊙ and M sec= 0.273 ± 0.002M ⊙, and radii R WD= 0.0168 ± 0.0003R ⊙ and R sec= 0.306 ± 0.007R ⊙. For GKVir, we find an inclination of i= 895°± 06, masses of M WD= 0.564 ± 0.014M ⊙ and M sec= 0.116 ± 0.003M ⊙ and radii R WD= 0.0170 ± 0.0004R ⊙ and R sec= 0.155 ± 0.003R ⊙. The mass and radius of the WD in GKVir are consistent with evolutionary models for a 50000K carbon-oxygen (CO) core WD. Although the mass and radius of the WD in SDSSJ1212-0123 are consistent with CO core models, evolutionary models imply that a WD with such a low mass and in a short period binary must have a helium core. The mass and radius measurements are consistent with helium core models but only if the WD has a very thin hydrogen envelope (M H/M WD≤ 10 -6). Such a thin envelope has not been predicted by any evolutionary models. The mass and radius of the secondary star in GKVir are consistent with evolutionary models after correcting for the effects of irradiation by the WD. The secondary star in SDSSJ1212-0123 has a radius ~9per cent larger than predicted. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS.