BAT99-9 -- a WC4 Wolf-Rayet star with nitrogen emission: Evidence for
  binary evolution?
  An analysis of the Large Magellanic Cloud (LMC) WC4 star BAT99-9 (HD 32125,
FD 4, Brey 7, WS 3) shows that the star still contains photospheric nitrogen.
Three N emission features (N V $\lambda\lambda 1238,1242$, N IV $\lambda 1719$,
N IV $\lambda\lambda 3479 - 3485$) are unambiguously identified in the
spectrum. CMFGEN models of the star yield a N/C ratio of $0.004 \pm 0.002$ (by
number) and a C/He ratio of $0.15_{-0.05}^{+0.10}$. Due to the similarity of
BAT99-9 to other WC4 stars, and the good fit achieved by CMFGEN to both the
classic WC4 spectrum, and the N lines, we argue that the N lines are intrinsic
to BAT99-9. An examination of a limited set of rotating models for single star
evolution at LMC and Galactic metallicities shows that a model with a Galactic
metallicity ($z=0.014$) and a progenitor mass of around $50\,M_\odot$ can have
a N/C ratio similar to, or larger than, what we observe for a significant
fraction of its lifetime. However, the LMC models ($z=0.006$) are inconsistent
with the observations. Both the single and binary BPASS models predict that
many WC stars can have a N/C ratio similar to, or larger than, what we observe
for a significant fraction of their lifetime. While the binary models cover a
wider range of luminosities and provide a somewhat better match to BAT99-9, it
is not currently possible to rule out BAT99-9 being formed through single star
evolution, given the uncertainties in mass-loss rates, and the treatment of
convection and mixing processes.
