16286

We propose to obtain low resolution ultraviolet spectroscopy of the newly identified 1201 s orbital period detached binary J2322+0509. This is the third shortest-period detached binary known comma and it contains two He-core white dwarfs at a virgul27 degree inclination. The low-inclination results in no visible photometric effects (e.g. comma eclipses comma ellipsoidal variations) comma but makes this one of the strongest sources of gravitational waves in the mHz frequency range. Located 760 pc from the Sun comma this binary has an estimated LISA 4-yr signal-to-noise ratio of 40. J2322+0509 is the first He+He white dwarf LISA verification binary comma a source class that is predicted to account for one-third of all LISA ultra-compact binary detections. Here we request HST STIS observations in the far-ultraviolet to constrain the physical parameters of this binary precisely. The optical data on this system reveals about 15% contribution from the cooler secondary white dwarf comma limiting the precision of our model atmosphere analysis on the primary white dwarf. The primary star is hot comma and it dominates in the far-ultraviolet. Our STIS data will enable us to constrain the temperature and mass of the primary precisely comma which in turn will constrain the radius and mass of the secondary star comma and the inclination of the system. Well-measured white dwarf binaries will be among the best electromagnetic + gravitational wave laboratories for understanding the formation of compact objects comma dynamical interactions and tides in binaries. J2322+0509 is one of the best such systems.