2014MNRAS.438.3399B

We present ULTRACAM photometry and X-Shooter spectroscopy of the eclipsing double white dwarf binary CSS 41177, the only such system that is also a double-lined spectroscopic binary. Combined modelling of the light curves and radial velocities yield masses and radii for both white dwarfs without the need to assume mass-radius relations. We find that the primary white dwarf has a mass of M1 = 0.38±0.02 M_☉ and a radius of R1 = 0.0222±0.0004 R_☉. The secondary white dwarf's mass and radius are M2 = 0.32±0.01 M_☉ and R2 = 0.0207±0.0004 R_☉, and its temperature and surface gravity (T2 = 11678±313K, log(g2) = 7.32±0.02) put it close to the white dwarf instability strip. However, we find no evidence for pulsations to roughly 0.5 percent relative amplitude. Both masses and radii are consistent with helium white dwarf models with thin hydrogen envelopes of ≤ 10^–4 M*. The two stars will merge in 1.14±0.07Gyr due to angular momentum loss via gravitational wave emission.