  Nitrogen isotopic ratios are a key tool for tracing Galactic stellar
nucleosynthesis. We present the first study of the $^{14}$N/$^{15}$N abundance
ratio in the outer regions of the Milky Way (namely, for galactocentric
distances, $R_{\rm GC}$, from 12 kpc up to 19 kpc), with the aim to study the
stellar nucleosynthesis effects in the global Galactic trend. We analysed IRAM
30m observations towards a sample of 35 sources in the context of the CHEMical
complexity in star-forming regions of the OUTer Galaxy (CHEMOUT) project. We
derived the $^{14}$N/$^{15}$N ratios from HCN and HNC for 14 and 3 sources,
respectively, using the $J$ = 1-0 rotational transition of HN$^{13}$C,
H$^{15}$NC, H$^{13}$CN, and HC$^{15}$N. The results found in the outer Galaxy
have been combined with previous measurements obtained in the inner Galaxy. We
find an overall linear decreasing H$^{13}$CN/HC$^{15}$N ratio with increasing
$R_{\rm GC}$. This translates to a parabolic $^{14}$N/$^{15}$N ratio with a
peak at 11 kpc. Updated Galactic chemical evolution models have been taken into
account and compared with the observations. The parabolic trend of the
$^{14}$N/$^{15}$N ratio with $R_{\rm GC}$ can be naturally explained (i) by a
model that assumes novae as the main $^{15}$N producers on long timescales
($\ge$1 Gyr) and (ii) by updated stellar yields for low- and intermediate-mass
stars.
