  Binaries consisting of a hot subdwarf star and an accreting white dwarf (WD)
are sources of gravitational wave radiation at low frequencies and possible
progenitors of type Ia supernovae if the WD mass is large enough. Here, we
report the discovery of the third binary known of this kind: it consists of a
hot subdwarf O (sdO) star and a WD with an orbital period of 3.495 hours and an
orbital shrinkage of 0.1 s in 6 yr. The sdO star overfills its Roche lobe and
likely transfers mass to the WD via an accretion disk. From spectroscopy, we
obtain an effective temperature of $T_{\mathrm{eff}}=54\,240\pm1\,840$ K and a
surface gravity of $\log{g}=4.841\pm0.108$ for the sdO star. From the light
curve 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 the
disk 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, where
the progenitor of the sdO star is either an RGB star or, more likely, an early
AGB star; the sdO star will subsequently evolve into a WD and merge with its WD
companion, likely resulting in an R CrB star. The outstanding feature in the
spectrum 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 a
density $\sim 6\times 10^{-10} {\rm g/cm^3}$.
