  Early-type R stars and J stars are a special type of carbon star, having
enhanced nitrogen ($\rm [N/Fe]\approx 0.5$), lithium, a low
\iso{12}{C}/\iso{13}{C} ratio ($<15$) and no s-element enhancements. The merger
of a helium white dwarf with a red giant is regarded to be a possible model for
the origin of early-type R stars, but the details of nucleosynthesis are not
clear. In this paper we investigate three possible channels for helium
white-dwarf + red-giant mergers, and find that, amongst the three, only a
high-mass helium white dwarf subducted into a low core-mass red giant can make
an early-type R star. Nucleosynthesis of elements carbon, nitrogen, oxygen and
lithium correspond well with the observations. Furthermore, we find that the J
stars may represent a short and luminous stage in the evolution of an early-R
star.
