Ring Nebulae: Tracers of the CNO Nucleosynthesis
  Preliminary results are presented from spectroscopic data in the optical
range of the Galactic ring nebulae NGC 6888, G2:4+1:4, RCW 58 and Sh2-308. Deep
observations with long exposure times were carried out at the 6.5m Clay
Telescope and at the 10.4m Gran Telescopio Canarias. In NGC 6888, recombination
lines of C II, O II and N II are detected with signal-to-noise ratios higher
than 8. The chemical content of NGC 6888 is discussed within the chemical
enrichment predicted by evolution models of massive stars. For all nebulae, a
forthcoming work will content in-depth details about observations, analysis and
final results (Esteban et al. 2015, in prep.).
