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We present spectrophotometric observations of a sample of 80 southern galactic planetary nebulae (PN), and derive elemental abundances for 68 objects, supplementing the optical observations with UV data in 25 cases. We define Type I PN as those objects that have experienced envelope-burning conversion to nitrogen of dredgedup primary carbon. Such nebulae are recognised by their having nitrogen abundances that exceed the total C + N abundance of H II regions in the same galaxy. In our own galaxy, this criterion corresponds to N/O > 0. 8. In the current sample, 11 nebulae having N/O > 0. 8 are thereby classified as Type I. For these Type I PN, no evidence is found for oxygen depletion, compared with non-Type I PN. No trend is found between the N/O and O/H ratios for the entire sample, and the mean O/H ratios for the non-Type I and Type I PN are the same within the errors; O/H= (4. 93±2. 22) ×10−4 by number for 42 non-Type I PN and O/H= (4. 42±1. 44) ×10−4 for 11 Type I PN. Also, no difference is found between the oxygen abundances in the PN in this sample and the oxygen abundances in galactic H II regions. Hence we find no evidence for the ON cycle (which is predicted to operate during the second dredge-up) to have significantly altered the surface abundances of the progenitor stars, even for the Type I PN. helium abundances derived for the non-Type I PN are in accord with those predicted by Becker the carbon has been enhanced by He-burning processed material brought up by the third dredge-up.
Kingsburgh et al. (Tue,) studied this question.