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Binaries consisting of a hot subdwarf star and an accreting white dwarf (WD)are sources of gravitational wave radiation at low frequencies and possibleprogenitors of type Ia supernovae if the WD mass is large enough. Here, wereport the discovery of the third binary known of this kind: it consists of ahot subdwarf O (sdO) star and a WD with an orbital period of 3.495 hours and anorbital shrinkage of 0.1 s in 6 yr. The sdO star overfills its Roche lobe andlikely transfers mass to the WD via an accretion disk. From spectroscopy, weobtain an effective temperature of $T_{\mathrm{eff}}=54\,240\pm1\,840$ K and asurface gravity of $\log{g}=4.841\pm0.108$ for the sdO star. From the lightcurve analysis, we obtain a sdO mass of $M_{\mathrm{sdO}}=0.55$${\mathrm{M_{\odot}}}$ and a mass ratio of$q=M_{\mathrm{WD}}/M_{\mathrm{sdO}}=0.738\pm0.001$. Also, we estimate that thedisk has a radius of $\sim 0.41R_\odot$ and a thickness of $\sim 0.18R_\odot$.The origin of this binary is probably a common envelope ejection channel, wherethe progenitor of the sdO star is either an RGB star or, more likely, an earlyAGB star; the sdO star will subsequently evolve into a WD and merge with its WDcompanion, likely resulting in an R CrB star. The outstanding feature in thespectrum of this object is strong Ca H&K lines, which are blueshifted by$\sim$200 km/s and likely originate from the recently ejected common envelope,and we estimated that the remnant CE material in the binary system has adensity $\sim 6\times 10^{-10} {\rm g/cm^3}$.

A Roche Lobe-filling hot Subdwarf and White Dwarf Binary: possible detection of an ejected common envelope