2020ApJ...898L..25K

We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of T_eff = 33,700 ± 1000 K and a surface gravity of log(g) = 5.54 ± 0.11. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of M_sdOB = 0.41 ± 0.04 M_☉ and a WD mass of M_WD = 0.68 ± 0.05 M_☉. The weak eclipses imply a WD blackbody temperature of 63,000 ± 10,000 K and a radius R_WD = 0.0148 ± 0.0020 R_☉ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for ≃1 Myr at a rate of 10^–9 M_☉ yr^–1, which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in ≃30 Myr. We suggest that Galactic reddening could bias discoveries toward preferentially finding Roche lobe-filling systems during the short-lived shell-burning phase. Studies using reddening-corrected samples should reveal a large population of helium core-burning hot subdwarfs with T_eff ≃ 25,000 K in binaries of 60-90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational-wave emission and explode as a subluminous thermonuclear supernova or evolve into a massive single WD.