  We propose a formation channel for the previously unexplained helium-rich
subdwarf O (He-rich sdO) stars in which post-subdwarf B (sdB) stars (i.e.
hybrid COHe white dwarfs) reignite helium burning in a shell after gaining
matter from their helium white-dwarf (WD) companions. Such short-period
binaries containing post-sdB WDs and helium WDs are predicted by one of the
major binary formation channels for sdB stars. In the majority of cases, mass
transfer is expected to lead to a dynamically unstable merger event, leaving a
single-star remnant. Calculations of the evolution of these stars show that
their properties are consistent with the observed He-rich sdO stars. The
luminosity of these stars is about an order of magnitude higher than that of
canonical sdB stars. We also suggest that binary systems such as PG 1544+488
(Ahmad et al. 2004) and HE 0301-3039 (Lisker et al. 2004), which each contain
two hot subdwarfs, could be the outcome of a double-core common-envelope phase.
Since this favours intermediate-mass progenitors, this may also explain why the
subdwarfs in these systems are He-rich.
