  We study the role played both by novae and single stars in enriching the ISM
of the Galaxy with CNO group nuclei, in the framework of a detailed successful
model for the chemical evolution of both the Galactic halo and disc. Once all
the nucleosynthesis sources of CNO elements are taken into account, we conclude
that 13C, 15N and 17O are likely to have both a primary and a secondary origin,
in contrast to previous beliefs. Given the uncertainties still present in the
computation of theoretical stellar yields, our results can be used to put
constraints on stellar evolution and nucleosynthesis models.
