* Astronomy

Title: A deeply eclipsing detached double helium white dwarf binary
Authors: S. G. Parsons, T. R. Marsh, B. T. Gänsicke, A. J. Drake, D. Koester

Using Liverpool Telescope+RISE photometry we identify the 2.78 hour period binary star CSS 41177 as a detached eclipsing double white dwarf binary with a 21,100K primary star and a 10,500K secondary star. This makes CSS 41177 only the second known eclipsing double white dwarf binary after NLTT 11748. The 2 minute long primary eclipse is 40% deep and the secondary eclipse 10% deep. From Gemini+GMOS spectroscopy we measure the radial velocities of both components of the binary from the H{\alpha} absorption line cores. These measurements, combined with the light curve information, yield white dwarf masses of M1 = 0.283  ±0.064Msun and M2 = 0.274 ±0.034Msun, making them both helium core white dwarfs. As an eclipsing, double-lined spectroscopic binary CSS 41177 is ideally suited to measuring precise, model-independent masses and radii. The two white dwarfs will merge in roughly 1.1 Gyr to form a single sdB star.

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