DISCOVERY OF THE ECLIPSING DETACHED DOUBLE WHITE DWARF BINARY NLTT 11748

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 (approximate to 1 minute) differential photometry with the Las Cumbres Observatory's Faulkes Telescope North. Rather than pulsations, we discovered approximate to 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 K-1 = 271 +/- 3 kms(-1) and confirmed the dips as eclipses caused by an orbiting WD with a mass M-2 = 0.648-0.771 M-circle dot for M-1 = 0.1-0.2M(circle dot). 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 R-1 = 0.043-0.039 R-circle dot for M-1 = 0.1-0.2M(circle dot), consistent with the theoretically expected values for a helium core WD with a thick, stably burning hydrogen envelope. Though nearby (at approximate to 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 M-1, yielding accurate WD mass-radius measurement of both components, as well as a clearer indication of the binary's fate once contact is reached.