The first ultracompact Roche lobe-filling hot subdwarf binary

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 temperature of = 42,400 300 K and a surface gravity of M-sdOB = 0.337 0.015 M-WD = 0.545 0.020 . Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5-2.8 star lost its envelope when crossing the Hertzsprung gap. To match its current and masses, we estimate a post-common envelope period of 150 minutes and find that the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of 0.1, merge with its carbon/oxygen white dwarf companion after 17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.