2010ApJ...716L.146S

We report the discovery of the first eclipsing detached double white dwarf (WD) binary. In a pulsation search, the low-mass helium core WD NLTT 11748 was targeted for fast (≈ 1 minute) differential photometry with the Las Cumbres Observatory's Faulkes Telescope North. Rather than pulsations, we discovered ≈ 180 s 3%-6% dips in the photometry. Subsequent radial velocity measurements of the primary white dwarf from the Keck telescope found variations with a semi-amplitude K₁= 271±3 km/s and confirmed the dips as eclipses caused by an orbiting WD with a mass M₂= 0.648-0.771 M_☉ for M₁= 0.1-0.2 M_☉. We detect both the primary and secondary eclipses during the P_orb= 5.64 hr orbit and measure the secondary's brightness to be 3.5%±0.3% of the primary at SDSS-g'. Assuming that the secondary follows the mass-radius relation of a cold C/O WD and including the effects of microlensing in the binary, the primary eclipse yields a primary radius of R₁= 0.043-0.039 R_☉ for M₁= 0.1-0.2 M_☉, consistent with the theoretically expected values for a helium core WD with a thick, stably burning hydrogen envelope. Though nearby (at ≈ 150 pc), the gravitational wave strain from NLTT 11748 is likely not adequate for direct detection by the Laser Interferometer Space Antenna. Future observational efforts will determine M₁, yielding accurate WD mass-radius measurement of both components, as well as a clearer indication of the binary's fate once contact is reached.