2017ApJ...851...28K

We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW J074106.0-294811.0 with an orbital period of P_orb=44.66279±1.16×10^–4 minutes, 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, T_eff = 39 400±500 K and logg = 5.74 ± 0.09. High signal-to-noise ratio light curves show strong ellipsoidal modulation resulting in a derived sdOB mass M_sdOB=0.23±0.12M_☉ with a WD companion (M_WD=0.72±0.17M_☉). The mass ratio was found to be q=M_sdOB/M_WD=0.32±0.10. The derived mass for the He-sdOB is inconsistent with the canonical mass for hot subdwarfs of ~ 0.47M_☉. To put constraints on the structure and evolutionary history of the sdOB star we compared the derived T_eff, 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, which 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 minutes. 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.