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