We report the discovery of a detached double white dwarf binary with an orbital period of \approx 20.6 minutes , PTF J053332.05+020911.6 .
The visible object in this binary , PTF J0533+0209B , is a \approx 0.17 M _ { \odot } mass white dwarf with a helium-dominated atmosphere containing traces of hydrogen ( DBA ) .
This object exhibits ellipsoidal variations due to tidal deformation , and is the visible component in a single-lined spectroscopic binary with a velocity semi-amplitude of K _ { B } = 618.7 \pm 6.9 km s ^ { -1 } .
We have detected significant orbital decay due to the emission of gravitational radiation , and we expect that the Laser Interferometer Space Antenna ( LISA ) \citep 2017arXiv170200786A will detect this system with a signal to noise of 8.4 ^ { +4.2 } _ { -3.0 } after four years of operation .
Because this system already has a well determined orbital period , radial velocity semi-amplitude , temperature , atmospheric composition , surface gravity , and orbital decay rate , a LISA signal will help fully constrain the properties of this system by providing a direct measurement of its inclination .
Thus , this binary demonstrates the synergy between electromagnetic and gravitational radiation for constraining the physical properties of an astrophysical object .