A deeply eclipsing detached double helium white dwarf binary

Abstract

Using Liverpool Telescope+RISE photometry we identify the 2.78hr 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 CSS41177 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 Hα absorption line cores. These measurements, combined with the light curve information, yield white dwarf masses of M 1 = 0.283 ± 0.064 M⊙ and M 2 = 0.274 ± 0.034 M⊙, 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. © 2011. The American Astronomical Society. All rights reserved.