  We have detected circumstellar absorption lines of the $^{12}$CN and
$^{13}$CN Violet and Red System in the spectrum of the post-AGB star HD~56126.
From a synthetic spectrum analysis, we derive a Doppler broadening parameter of
$b=0.51\pm0.04$ km~s$^{-1}$, $^{12}$CN/$^{13}$CN$=38\pm2$, and a lower limit of
$2000$ on $^{12}$CN/$^{14}$CN and $^{12}$C$^{14}$N/$^{12}$C$^{15}$N. A simple
chemical model has been computed of the circumstellar shell surrounding
HD~56126 that takes into account the gas-phase ion-molecule reaction between CN
and C$^{+}$. From this we infer that this reaction leads to isotopic
fractionation of CN. Taking into account the isotopic exchange reaction and the
observed $^{12}$CN/$^{13}$CN we find $^{12}$C/$^{13}$C$\sim 67$ (for $T_{\rm
kin}=25$ K). Our analysis suggests that $^{12}$CN has a somewhat higher
rotational temperature than $^{13}$CN: $T_{\rm rot}=11.5\pm0.6$ and $8.0\pm0.6$
K respectively. We identify possible causes for this difference in excitation
temperature, among which the $N''$ dependence of the isotopic exchange
reaction.
