  We report the discovery of a new detached, double white dwarf system with an
orbital period of 39.8 min. We targeted SDSS J163030.58+423305.8 (hereafter
J1630) as part of our radial velocity program to search for companions around
low-mass white dwarfs using the 6.5m MMT. We detect peak-to-peak radial
velocity variations of 576 km/s. The mass function and optical photometry rule
out main-sequence companions. In addition, no milli-second pulsar companions
are detected in radio observations. Thus the invisible companion is most likely
another white dwarf. Unlike the other 39 min binary SDSS J010657.39-100003.3,
follow-up high speed photometric observations of J1630 obtained at the McDonald
2.1m telescope do not show significant ellipsoidal variations, indicating a
higher primary mass and smaller radius. The absence of eclipses constrain the
inclination angle to <82deg. J1630 contains a pair of white dwarfs, 0.3 Msun
primary + >0.3 Msun invisible secondary, at a separation of >0.32 Rsun. The two
white dwarfs will merge in less than 31 Myr. Depending on the core composition
of the companion, the merger will form either a single core-He burning subdwarf
star or a rapidly rotating massive white dwarf. The gravitational wave strain
from J1630 is detectable by instruments like the Laser Interferometer Space
Antenna (LISA) within the first year of operation.
