Search

Hot subdwarfs are early-type low-mass stars lying between the main sequence and the white dwarf sequence. The majority have helium-burning cores and helium-poor surfaces. A minority have helium-rich surfaces. A few, including Ton S 415, have surfaces with between 10 per cent and 90  per cent helium (by number). Their properties are diverse and their origins mostly unknown. Ton S 415 was identified as a suspected binary in a Southern African Large Telescope (SALT) survey of hot subdwarfs from its large radial velocity. Using SALT spectroscopy and Transiting Exoplanet Survey Satellite light curves, we confirm that Ton S 415 is a close binary with an orbital period |$p=84.6460\pm 0.0004\, {\rm min}$| and velocity semi-amplitude |$K=175.5\pm 1.0\, {\rm km\, s^{-1}}$|⁠. Analysis of the SALT spectrum and broad-band spectral energy distribution shows the visible star to have an effective temperature |$T_{\rm eff}=43\, 300\pm 1000\, {\rm K}$|⁠ , surface gravity |$\log g/{\rm cm\, s^{-2}}=5.89\pm 0.10$|⁠ , surface helium-to-hydrogen ratio log  y  = −0.62 ± 0.10, radius |$R=0.1074\pm 0.0025\, R_{\odot }$|⁠ , and mass |$M=0.33\pm 0.09\, {\rm {\rm M}_{\odot }}$|⁠. By modelling the light curve, we estimate the binary mass ratio to be q  = 0.7 ± 0.3 and infer an unseen white dwarf companion with a mass of |$M_{\rm WD}=0.47\pm 0.24\, {\rm {\rm M}_{\odot }}$|⁠. The results are consistent with a post-common-envelope binary that lost its hydrogen envelope before reaching the peak of the red giant branch, becoming a hot subdwarf with a non-canonical mass. We predict that the system will evolve into a double white dwarf binary before eventually either forming a stably accreting AM CVn system, or merging to form an R CrB star. [ABSTRACT FROM AUTHOR]