MEASUREMENTS OF THE C-12 C-13 RATIO IN PLANETARY-NEBULAE AND IMPLICATIONS FOR STELLAR EVOLUTION/

0We present the results of a study aimed at determining the C-12/C-13 ratio in two samples of planetary nebulae (PNe) by means of mm-wave ob servations of (CO)-C-12 and (CO)-C-13. The first group includes six PN e which have been observed in the He-3(+) hyperfine transition; the ot her group consists of 23 nebulae with rich molecular envelopes. We hav e determined the isotopic ratio in 14 objects and the results indicate a range of values between 9 and 23. In particular, three PNe have rat ios well below the value predicted by standard evolutionary models (gr eater than or similar to 20), indicating that some extra-mixing proces s has occurred in these stars. We briefly discuss the implications of our results fbr standard and nonstandard stellar nucleosynthesis.