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 (≈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 K 1 = 271 ± 3 km s −1 and confirmed the dips as eclipses caused by an orbiting WD with a mass M 2 = 0.648–0.771 M ☉ for M 1 = 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 R 1 = 0.043–0.039 R ☉ for M 1 = 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 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.