The First Ultracompact Roche Lobe–Filling Hot Subdwarf Binary

  title={The First Ultracompact Roche Lobe–Filling Hot Subdwarf Binary},
  author={Thomas Kupfer and Evan B. Bauer and Thomas R. Marsh and J. van Roestel and Eric C. Bellm and Kevin B Burdge and Michael W. Coughlin and Jim Fuller and J. J. Hermes and Lars Bildsten and Shrinivas R. Kulkarni and Thomas. A. Prince and Paula Szkody and Vik S. Dhillon and Gabriel Murawski and Rick S. Burruss and Richard Dekany and Alexandre Delacroix and Andrew J. Drake and D. Duev and Michael Feeney and Matthew J. Graham and David L. Kaplan and Russ R. Laher and Stuart P. Littlefair and Frank J. Masci and Reed L. Riddle and Ben Rusholme and Eugene Serabyn and Roger M. Smith and David L. Shupe and Maayane T. Soumagnac},
  journal={The Astrophysical Journal},
We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic plane named the Zwicky Transient Facility and exhibits a period of P = 39.3401(1) minutes, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective… 
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