  We have compiled composition, luminosity, and binarity information for
carbon-enhanced, metal-poor (CEMP) stars reported by recent studies. We divided
the CEMP star sample into two classes, having high and low abundances,
respectively, of the s-process elements, and consider the abundances of several
isotopes, in particular 12C, 13C, and 14N, as well as the likely evolutionary
stages of each star. Despite the fact that objects in both groups were selected
from the same surveys (primarily the HK survey), without a-priori knowledge of
their s-process element abundances, we identify the following remarkable
difference between the two classes: s-element-rich CEMP (CEMP-s) stars occupy a
wide range of evolutionary states, but do not have a strongly evolved 13C/14N
ratio, whereas s-element-normal CEMP stars (CEMP-no) are found only high up the
first-ascent giant branch, and possess 13C/14N ratios approaching the CN-cycle
equilibrium value. Based on these observational facts, we discuss scenarios of
formation of CEMP stars.

Astrophysics