Precise parameters for both white dwarfs in the eclipsing binary CSS 41177

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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.