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We report the discovery of the ultracompact hot subdwarf (sdOB) binary OWJ074106.0-294811.0 with an orbital period of P$_{\rmorb}=44.66279\pm1.16\times10^{-4}$ min, making it the most compact hot subdwarfbinary known. Spectroscopic observations using the VLT, Gemini and Kecktelescopes revealed a He-sdOB primary with an intermediate helium abundance,T$_{\rm eff}=39 400\pm500$ K and log(g)=$5.74\pm0.09$. High signal-to-noiseratio lightcurves show strong ellipsoidal modulation resulting in a derivedsdOB mass $M_{\rm sdOB}=0.23\pm0.12$ M$_\odot$ with a WD companion ($M_{\rmWD}=0.72\pm0.17$ M$_\odot$). The mass ratio was found to be $q = M_{\rmsdOB}/M_{\rm WD}=0.32\pm0.10$. The derived mass for the He-sdOB is inconsistentwith the canonical mass for hot sbudwarfs of $\approx0.47$ M$_\odot$.  To put constraints on the structure and evolutionary history of the sdOB starwe compared the derived T$_{\rm eff}$, log(g) and sdOB mass to evolutionarytracks of helium stars and helium white dwarfs calculated with Modules forExperiments in Stellar Astrophysics (MESA). We find that the best fitting modelis a helium white dwarf with a mass of $0.320$ M$_\odot$, which left the commonenvelope ${\approx}1.1$ Myr ago, is consistent with the observations. As ahelium white dwarf with a massive white dwarf companion the object will reachcontact in 17.6 Myr at an orbital period of 5 min. Depending on the spin-orbitsynchronization timescale the object will either merge to form an R CrB star orend up as a stably accreting AM CVn-type system with a helium white dwarfdonor.

astrophysical journal/the astrophysical journal

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