  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 (approx 1 minute) differential photometry with the Las
Cumbres Observatory's Faulkes Telescope North. Rather than pulsations, we
discovered approx 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, and confirmed the dips
as eclipses caused by an orbiting WD with a mass M_2 = 0.648-0.771 M_sun for
M_1 = 0.1-0.2 M_sun. 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_sun for M_1 = 0.1-0.2 M_sun, consistent with the theoretically expected
values for a helium core WD with a thick, stably burning hydrogen envelope.
Though nearby (at approx 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.
