Orbital Decay in a 20 Minute Orbital Period Detached Binary with a Hydrogen-poor Low-mass White Dwarf

We report the discovery of a detached double white dwarf binary with an orbital period of 20.6 minutes, PTF J053332.05+020911.6. The visible object in this binary, PTF J0533+0209B, is a 0.17 M
K.B.B. thanks the National Aeronautics and Space Administration and the Heising-Simons Foundation for supporting his research. J.F. acknowledges support from an Innovator grant from The Rose Hills Foundation and the Sloan Foundation through grant FG-2018-10515. The KPED team thanks the National Science Foundation and the National Optical Astronomical Observatory for making the Kitt Peak 2.1 m telescope available. The KPED team thanks the National Science Foundation, the National Optical Astronomical Observatory and the Murty family for support in the building and operation of KPED. In addition, they thank the CHIMERA project for use of the Electron Multiplying CCD (EMCCD). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The research leading to these results has received funding from the European Research Council under the European Unions Horizon 2020 research and innovation program n.677706 (WD3D) This research benefited from interactions at the ZTF Theory Network Meeting that were funded by the Gordon and Betty Moore Foundation through grant GBMF5076 and support from the National Science Foundation through PHY-1748958.