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The early-R stars are carbon-rich K-type giants. They are enhanced in C12,C13 and N14, have approximately solar oxygen, magnesium isotopes, s-process andiron abundances, have the luminosity of core-helium burning stars, are notrapid rotators, are members of the Galactic thick disk and, most peculiarly ofall, are all single stars. Conventional single-star stellar evolutionary modelscannot explain such stars, but mergers in binary systems have been proposed toexplain their origin.  We have synthesized binary star populations to calculate the number of mergedstars with helium cores which could be early-R stars. We find many possibleevolutionary channels. The most common of which is the merger of a helium whitedwarf with a hydrogen-burning red giant branch star during a common envelopephase followed by a helium flash in a rotating core which mixes carbon to thesurface. All the channels together give ten times more early-R stars than werequire to match recent Hipparcos observations - we discuss which channels arelikely to be the true early-R stars and which are not. For the first time wehave constructed a viable model of the early-R stars with which we can testsome of our ideas regarding common envelope evolution in giants, stellarmergers, rotation, the helium flash and the origin of the early-R stars.

Origin of the early-type R stars: a binary-merger solution to a century-old problem?