  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(2) Msun and a radius of R1 = 0.0222(4) Rsun. The secondary
white dwarf's mass and radius are M2 = 0.32(1) Msun and R2 = 0.0207(4) Rsun,
and its temperature and surface gravity (T2 = 11678(313) K, log(g2) = 7.32(2))
put it close to the white dwarf instability strip. However, we find no evidence
for pulsations to roughly 0.5% relative amplitude. Both masses and radii are
consistent with helium white dwarf models with thin hydrogen envelopes of
0.0001 Mstar. The two stars will merge in 1.14 Gyr due to angular momentum loss
via gravitational wave emission.
