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

Discovery of hot subdwarfs with extremely high carbon and oxygen abundances