Mónica Rodríguez ( INAOE, Mexico)
The oxygen abundances of H II regions and planetary nebulae (PNe) provide crucial clues for our understanding of galactic chemical evolution, especially when they are compared with each other and with the abundances derived for stars of different ages. However, the nebular oxygen abundances are critically uncertain, since the abundances we derive from oxygen recombination lines are larger than those derived from collisionally excited lines by factors around two or more. This abundance discrepancy could be due to the presence within the nebulae of metal-rich inclusions, composed of stellar ejecta, supernova ejecta, or planetary material. On the other hand, the discrepancy might arise from errors in the recombination coefficients or in the assumed temperature structure of the nebulae. I will discuss these possibilities in the context of a comparison of the resulting abundances with those derived using completely different techniques for stars and the diffuse interstellar medium.