A new gravitational wave verification source

We report the discovery of a detached 20-min orbital period binary white dwarf (WD). 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 8 m Gemini and 6.5 m MMT reveal peak-to-peak radial velocity variations of a parts per thousand 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 2.1 m 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 hierarchical triple system. WD 0931+444 contains a pair of WDs, a 0.32 M-aS (TM) primary and a a parts per thousand yen0.14 M-aS (TM) secondary, at a separation of a parts per thousand yen0.19 R-aS (TM). 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 a parts per thousand currency sign9 Myr. The log h a parts per thousand integral -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.