The chemical composition of Galactic ring nebulae around massive stars
  We present deep spectra of ring nebulae associated with Wolf-Rayet (WR) and
O-type stars: NGC 6888, G2.4+1.4, RCW 58, S 308, NGC 7635 and RCW 52. The data
have been taken with the 10m Gran Telescopio Canarias and the 6.5m Clay
Telescope. We extract spectra of several apertures in some of the objects. We
derive C$^{++}$ and O$^{++}$ abundances from faint recombination lines in NGC
6888 and NGC 7635, permitting to derive their C/H and C/O ratios and estimate
the abundance discrepancy factor (ADF) of O$^{++}$. The ADFs are larger than
the typical ones of normal HII regions but similar to those found in the
ionised gas of star-forming dwarf galaxies. We find that chemical abundances
are rather homogeneous in the nebulae where we have spectra of several
apertures: NGC 6888, NGC 7635 and G2.4+1.4. We obtain very high values of
electron temperature in a peripheral zone of NGC 6888, finding that shock
excitation can reproduce its spectral properties. We find that all the objects
associated with WR stars show N enrichment. Some of them also show He
enrichment and O deficiency as well as a lower Ne/O than expected, this may
indicate the strong action of the ON and NeNa cycles. We have compared the
chemical composition of NGC 6888, G2.4+1.4, RCW 58 and S 308 with the
nucleosynthesis predicted by stellar evolution models of massive stars. We find
that non-rotational models of stars of initial masses between 25 and 40 solar
masses seem to reproduce the observed abundance ratios of most of the nebulae.
