  CEMP-s stars are long-lived low-mass stars with a very low iron content as
well as overabundances of carbon and s-elements. Their peculiar chemical
pattern is often explained by pollution from a AGB star companion. Recent
observations have shown that most of the CEMP-s stars are in a binary system,
providing support to the AGB companion scenario. A few CEMP-s stars, however,
appear to be single. We inspect four apparently single CEMP-s stars and discuss
the possibility that they formed from the ejecta of a previous-generation
massive star, referred to as the "source" star. In order to investigate this
scenario, we computed low-metallicity massive star models with and without
rotation and including complete s-process nucleosynthesis. We find that
non-rotating source stars cannot explain the observed abundance of any of the
four CEMP-s stars. Three out of the four CEMP-s stars can be explained by a
$25$ $M_{\odot}$ source star with $v_{\rm ini} \sim 500$ km s$^{-1}$
(spinstar). The fourth CEMP-s star has a high Pb abundance that cannot be
explained by any of the models we computed. Since spinstars and AGB predict
different ranges of [O/Fe] and [ls/hs], these ratios could be an interesting
way to further test these two scenarios.
