  Helium rich subdwarf O stars (sdOs) are hot compact stars in a pre-white
dwarf evolutionary state. Most of them have effective temperatures and surface
gravities in the range Teff = 40,000-50,000 K and log g = 5.5-6.0. Their
atmospheres are helium dominated. If present at all, C, N, and O are trace
elements. The abundance patterns are explained in terms of nucleosynthesis
during single star evolution (late helium core flash) or a binary He-core white
dwarf merger. Here we announce the discovery of two hot hydrogen-deficient sdOs
(PG1654+322 and PG1528+025) that exhibit unusually strong carbon and oxygen
lines. A non-LTE model atmosphere analysis of spectra obtained with the Large
Binocular Telescope and by the LAMOST survey reveals astonishingly high
abundances of C (~20%) and O (~20%) and that the two stars are located close to
the helium main sequence. Both establish a new spectroscopic class of hot
H-deficient subdwarfs (CO-sdO) and can be identified as the remnants of a
He-core white dwarf that accreted matter of a merging low-mass CO-core white
dwarf. We conclude that the CO-sdOs represent an alternative evolutionary
channel creating PG1159 stars besides the evolution of single stars that
experience a late helium-shell flash.
