We report the discovery of a detached 20 min orbital period binary white dwarf . WD 0931+444 ( SDSS J093506.93+441106.9 ) was previously classified as a WD + M dwarf system based on its optical spectrum . Our time-resolved optical spectroscopy observations obtained at the 8m Gemini and 6.5m MMT reveal peak-to-peak radial velocity variations of \approx 400 km s ^ { -1 } every 20 min for the WD , but no velocity variations for the M dwarf . In addition , high-speed photometry from the McDonald 2.1m telescope shows no evidence of variability nor evidence of a reflection effect . An M dwarf companion is physically too large to fit into a 20 min orbit . Thus , the orbital motion of the WD is almost certainly due to an invisible WD companion . The M dwarf must be either an unrelated background object or the tertiary component of a hiearchical triple system . WD 0931+444 contains a pair of WDs , a 0.32 M _ { \sun } primary and a \geq 0.14 M _ { \sun } secondary , at a separation of \geq 0.19 R _ { \sun } . After J0651+2844 , WD 0931+444 becomes the second-shortest period detached binary WD currently known . The two WDs will lose angular momentum through gravitational wave radiation and merge in \leq 9 Myr . The \log h \simeq - 22 gravitational wave strain from WD 0931+444 is strong enough to make it a verification source for gravitational wave missions in the milli-Hertz frequency range , e.g . the evolved Laser Interferometer Space Antenna ( eLISA ) , bringing the total number of known eLISA verification sources to nine .