Abstract
We present elemental abundances for a sample of 70 southern Galactic planetary nebulae (PN), based on observations at both UV and optical wavelengths. In this sample, 6 definite Type I objects are identified. For the Type I PN, no evidence for oxygen depletion compared to non-Type I PN is found, hence the ON cycle which operates in the 2nd dredge up does not significantly affect the surface abundances of the progenitor star. For the non-Type I PN, we find that the conversion of carbon into nitrogen during the first dredge-up phase is sufficient to account for the increase in nitrogen abundances over HII regions. A comparison between nitrogen abundances in PN and carbon+nitrogen abundances in Galactic HII regions indicates that roughly 30% of the initial carbon is converted into nitrogen for the non-Type I PN. However, in order to explain the high nitrogen abundances derived for Type I PN, convective envelope burning during the 3rd dredge-up phase is required. Total C+N+O abundances are found to be correlated with C/H for the combined non-Type I and Type I sample; the carbon abundance is increased via He-burning, the products of which are subsequently brought to the surface by the 3rd dredge-up
