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We present the results of a study aimed at determining the 12C/13C ratio intwo samples of planetary nebuale (PNe) by means of mm-wave observations of 12COand 13CO. The first group includes six PNe which have been observed in the 3He+hyperfine transition; the other group consists of 23 nebulae with richmolecular envelopes. We have determined the isotopic ratio in 14 objects andthe results indicate a range of values between 9 and 23. In particular, threePNe have ratios well below the value predicted by standard evolutionary models(about 20), indicating that some extra-mixing process has occurred in thesestars. We briefly discuss the implications of our results for standard andnonstandard stellar nucleosynthesis.

Measurements of the 12C/13C Ratio in Planetary Nebulae and Implications for Stellar Evolution