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 h 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 disc. From spectroscopy, we obtain an effective temperature of T-eff = 54 240 +/- 1840 K and a surface gravity of log g = 4.841 +/- 0.108 for the sdO star. From the light curve analysis, we obtain an sdO mass of M-sdO = 0.55 M-circle dot and a mass ratio of q = M-WD/M-sdO = 0.738 +/- 0.001. Also, we estimate that the disc has a radius of similar to 0.41 R-circle dot and a thickness of similar to 0.18 R-circle dot. The origin of this binary is probably a common envelope ejection channel, where the progenitor of the sdO star is either a red giant branch star or, more likely, an early asymptotic giant branch star; the sdO star will subsequently evolve into a WD and merge with its WD companion, likely resulting in an R Coronae Borealis (R CrB) star. The outstanding feature in the spectrum of this object is strong Ca H&K lines, which are blue-shifted by similar to 200 km s(-1) and likely originate from the recently ejected common envelope, and we estimated that the remnant common envelope (CE) material in the binary system has a density similar to 6 x 10(-1)0 g cm(-3).