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 (approx 1-min) 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 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 eclipse 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 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