  Some O and B stars show unusually strong or weak lines of carbon and/or
nitrogen. These objects are classified as OBN or OBC stars. It has recently
been shown that nitrogen enrichment and carbon depletion are the most likely
explanations for the existence of the ON class. We investigate OC stars (all
being supergiants) to check that surface abundances are responsible for the
observed anomalous line strengths. We perform a spectroscopic analysis of three
OC supergiants using atmosphere models. A fourth star was previously studied by
us. Our sample thus comprises all OC stars known to date in the Galaxy. We
determine the stellar parameters and He, C, N, and O surface abundances. We
show that all stars have effective temperatures and surface gravities fully
consistent with morphologically normal O supergiants. However, OC stars show
little, if any, nitrogen enrichment and carbon surface abundances consistent
with the initial composition. OC supergiants are thus barely chemically
evolved, unlike morphologically normal O supergiants.
