Discovery of the closest hot subdwarf binary with white dwarf companion

We report the discovery of an extremely close, eclipsing binary system. A white dwarf is orbited by a core He-burning compact hot subdwarf star with a period as short as $\simeq 0.04987\mathrm{d}$ making this system the most compact hot subdwarf binary discovered so far. The subdwarf will start to transfer helium-rich material on short timescales of less than $50\mathrm{M}\mathrm{y}\mathrm{r}$. The ignition of He-burning at the surface may trigger carbon-burning in the core although the WD is less massive than the Chandrasekhar limit ($>0.74,M_{\rm \odot}$) making this binary a possible progenitor candidate for a supernova type Ia event.