A 1201 s Orbital Period Detached Binary: the First Double Helium Core White Dwarf LISA Verification Binary

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We report the discovery of a 1201 s orbital period binary, the third shortest-period detached binary known.

• SDSS J232230.20+050942.06 contains two He-core white dwarfs orbiting with a 27 deg inclination.
• Located 0.76 kpc from the Sun, the 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, a source class that is predicted to account for one-third of resolved LISA ultra-compact binary detections.