| Summary: | In this thesis, I describe the evolution following the merger of two helium white dwarfs. Three sets of assumptions concerning the distribution of debris material between a disc and a corona been examined. The results demonstrate that most helium rich hot subdwarfs should be produced by helium white dwarf mergers, and some of those with high mass may then become RCrB and extreme helium stars. If this merger process happens between a high mass helium white dwarf and the core of a red-giant star during a common envelope phase, the post-mergers would resemble the early-type of R stars. Nucleosynthesis of the elements carbon, nitrogen, oxygen and lithium corresponds well with the observations. Furthermore, the simulations indicate that J. stars may represent a short and luminous stage in the formation of an early-type R star.
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