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Orbital decay by gravitational-wave radiation will cause some close-binarywhite dwarfs (WDs) to merge within a Hubble time. The results from previoushydrodynamical WD-merger simulations have been used to guide calculations ofthe post-merger evolution of carbon-oxygen + helium (CO+He) WD binaries. Ourmodels include the formation of a hot corona in addition to a Keplerian disk.We introduce a 'destroyeddisk' model to simulate the effect of direct diskingestion into the expanding envelope. These calculations indicate significantlifetimes in the domain of the rare R Coronae Borealis (RCB) stars, before afast evolution through the domain of the hotter extreme helium (EHe) stars.Surface chemistries of the resulting giants are in partial agreement with theobserved abundances of RCB and EHe stars. The production of 3He, 18O and 19Fare discussed. Evolutionary timescales combined with binary white-dwarf mergerrates from binary-star population synthesis are consistent with present-daynumbers of RCBs and EHes, provided that the majority come from relativelyrecent (< 2Gyr) star formation. However, most RCBs should be produced by CO-WD+ low-mass HeWD mergers, with the He-WD having a mass in the range 0.20-0.35Msolar. Whilst, previously, a high He-WD mass (> 0.40 Msolar) was required tomatch the carbon-rich abundances of RCB stars, the 'destroyed-disk' modelyields a high-carbon product with He-WD mass > 0.30 Msolar, in better agreementwith population synthesis results.

Post-merger evolution of carbon-oxygen + helium white dwarf binaries and the origin of R Coronae Borealis and extreme helium stars