12C/13C ratios in stars ascending the giant branch the first time.

Stellar evolution calculations were carried out for 1 M0 and 2 Mo red giants in order to understand the observed 12C!13C ratios. Although very low values of S to 7 in some red supergiants can be understood in terms of thermally unstable helium shell burning models, there remains a problem of moderately low values (10 to 20) in stars not luminous enough for this process. In addition to the standard mixing at the base of the giant branch, which can give 12C/13C ratios in the region of 25 to 30, we consider: variation in initial composition; meridional mixing; mass loss; zero-age composition gradient; mixing during the helium flash; and instability of the hydrogen-burning shell. Of these, only the mass loss mechanism and the hydrogen shell instability produce values below 20. Subject headings: stars: abundances - stars: late-type - stars: mass loss - stars: evolution