Due to orbital decay by gravitational-wave radiation , some close-binary helium white dwarfs are expected to merge within a Hubble time .
The immediate merger products are believed to be helium-rich sdO stars , essentially helium main-sequence stars .
We present new evolution calculations for these post-merger stars beyond the core helium-burning phase .
The most massive He-sdO ’ s develop a strong helium-burning shell and evolve to become helium-rich giants .
We include nucleosynthesis calculations following the merger of 0.4 M _ { \odot } helium white-dwarf pairs with metallicities Z = 0.0001 , 0.004 , 0.008 and 0.02 .
The surface chemistries of the resulting giants are in partial agreement with the observed abundances of R Coronae Borealis and extreme helium stars .
Such stars might represent a third , albeit rare , evolution channel for the latter , in addition to the CO+He white dwarf merger and the very-late thermal pulse channels proposed previously .
We confirm a recent suggestion that lithium seen in R CrB stars could form naturally during the hot phase of a merger in the presence of ^ { 3 } { He } from the donor white dwarf .