  Rotational-vibrational transitions of the fundamental vibrational modes of
the $^{12}$C$^{14}$N$^+$ and $^{12}$C$^{15}$N$^+$ cations have been observed
for the first time using a cryogenic ion trap apparatus with an action
spectroscopy scheme. The lines P(3) to R(3) of $^{12}$C$^{14}$N$^+$ and R(1) to
R(3) of $^{12}$C$^{15}$N$^+$ have been measured, limited by the trap
temperature of approximately 4 K and the restricted tuning range of the
infrared laser. Spectroscopic parameters are presented for both isotopologues,
with band origins at 2000.7587(1) and 1970.321(1) cm$^{-1}$, respectively, as
well as an isotope independent fit combining the new and the literature data.
