  Photoabsorption spectra of ${}^{14}{\rm N}{}^{15}{\rm N}$ were recorded at
high resolution with a vacuum-ultraviolet Fourier-Transform spectrometer fed by
synchrotron radiation in the range 81 to 100 nm. The combination of high column
density ($3\times 10^{17}$ cm$^{-2}$) and low temperature (98 K) allowed for
the recording of weak spin-forbidden absorption bands exciting levels of
triplet character. The triplet states borrow intensity from ${}^1\Pi_u$ states
of Rydberg and valence character while causing their predissociation. New
predissociation linewidths and molecular constants are obtained for the states
$C\,{}^3\Pi_u(v=7,8,14,15,16,21)$, $G\,{}^3\Pi_u(v=0,1,4)$, and
$F\,{}^3\Pi_u(v=0)$. The positions and widths of these levels are shown to be
well-predicted by a coupled-Schroedinger equation model with empirical
parameters based on experimental data on ${}^{14}{\rm N}_2$ and ${}^{15}{\rm
N}_2$ triplet levels.
