The Omegawhite Survey for Short-period Variable Stars - v. Discovery of an Ultracompact Hot Subdwarf Binary with a Compact Companion in a 44 Minute Orbit

We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW J074106.0-294811.0 with an orbital period of Porb=44.66279±1.16×10 min, making it the most compact hot subdwarf binary known. Spectroscopic observations using the VLT, Gemini and Keck telescopes revealed a He-sdOB primary with an intermediate helium abundance, Teff =39400±500 K and logg =5.74± 0.09. High signal-to-noise ratio lightcurves show strong ellipsoidal modulation resulting in a derived sdOB mass MsdOB = 0.23± 0.12 M with a WD companion (MWD = 0.72± 0.17 M ). The mass ratio was found to be q = MsdOB/MWD = 0.32±0.10. The derived mass for the He-sdOB is inconsistent with the canonical mass for hot sbudwarfs of ≈ 0.47 M . To put constraints on the structure and evolutionary history of the sdOB star we compared the derived Teff , logg and sdOB mass to evolutionary tracks of helium stars and helium white dwarfs calculated with Modules for Experiments in Stellar Astrophysics (MESA). We find that the best fitting model is a helium white dwarf with a mass of 0.320 M , which left the common envelope ≈1.1 Myr ago, is consistent with the observations. As a helium white dwarf with a massive white dwarf companion the object will reach contact in 17.6 Myr at an orbital period of 5 min. Depending on the spin-orbit synchronization timescale the object will either merge to form an R CrB star or end up as a stably accreting AM CVn-type system with a helium white dwarf donor.