  We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW
J074106.0-294811.0 with an orbital period of P$_{\rm
orb}=44.66279\pm1.16\times10^{-4}$ 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,
T$_{\rm eff}=39 400\pm500$ K and log(g)=$5.74\pm0.09$. High signal-to-noise
ratio lightcurves show strong ellipsoidal modulation resulting in a derived
sdOB mass $M_{\rm sdOB}=0.23\pm0.12$ M$_\odot$ with a WD companion ($M_{\rm
WD}=0.72\pm0.17$ M$_\odot$). The mass ratio was found to be $q = M_{\rm
sdOB}/M_{\rm WD}=0.32\pm0.10$. The derived mass for the He-sdOB is inconsistent
with the canonical mass for hot sbudwarfs of $\approx0.47$ M$_\odot$.
  To put constraints on the structure and evolutionary history of the sdOB star
we compared the derived T$_{\rm eff}$, log(g) 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$_\odot$, which left the common
envelope ${\approx}1.1$ 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.
