We report the discovery of a new detached , double white dwarf ( WD ) 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 WDs using the 6.5m MMT . We detect peak-to-peak radial velocity variations of 576 km s ^ { -1 } . 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 i \leq 82 ^ { \circ } . J1630 contains a pair of WDs , 0.3M _ { \sun } primary + \geq 0.3M _ { \sun } invisible secondary , at a separation of \geq 0.32R _ { \sun } . The two WDs 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 WD . The gravitational wave strain from J1630 is detectable by instruments like the Laser Interferometer Space Antenna ( LISA ) within the first year of operation .