  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 and
iron abundances, have the luminosity of core-helium burning stars, are not
rapid rotators, are members of the Galactic thick disk and, most peculiarly of
all, are all single stars. Conventional single-star stellar evolutionary models
cannot explain such stars, but mergers in binary systems have been proposed to
explain their origin.
  We have synthesized binary star populations to calculate the number of merged
stars with helium cores which could be early-R stars. We find many possible
evolutionary channels. The most common of which is the merger of a helium white
dwarf with a hydrogen-burning red giant branch star during a common envelope
phase followed by a helium flash in a rotating core which mixes carbon to the
surface. All the channels together give ten times more early-R stars than we
require to match recent Hipparcos observations - we discuss which channels are
likely to be the true early-R stars and which are not. For the first time we
have constructed a viable model of the early-R stars with which we can test
some of our ideas regarding common envelope evolution in giants, stellar
mergers, rotation, the helium flash and the origin of the early-R stars.
