The C$^{14}$N/C$^{15}$N Ratio in Diffuse Molecular Clouds
  We report the first detection of C$^{15}$N in diffuse molecular gas from a
detailed examination of CN absorption lines in archival VLT/UVES spectra of
stars probing local diffuse clouds. Absorption from the C$^{15}$N isotopologue
is confidently detected (at $\gtrsim4\sigma$) in three out of the four
directions studied and appears as a very weak feature between the main
$^{12}$CN and $^{13}$CN absorption components. Column densities for each CN
isotopologue are determined through profile fitting, after accounting for weak
additional line-of-sight components of $^{12}$CN, which are seen in the
absorption profiles of CH and CH$^+$ as well. The weighted mean value of
C$^{14}$N/C$^{15}$N for the three sight lines with detections of C$^{15}$N is
$274\pm18$. Since the diffuse molecular clouds toward our target stars have
relatively high gas kinetic temperatures and relatively low visual extinctions,
their C$^{14}$N/C$^{15}$N ratios should not be affected by chemical
fractionation. The mean C$^{14}$N/C$^{15}$N ratio that we obtain should
therefore be representative of the ambient $^{14}$N/$^{15}$N ratio in the local
interstellar medium. Indeed, our mean value agrees well with that derived from
millimeter-wave observations of CN, HCN, and HNC in local molecular clouds.
