Ja n 19 99 The Spectra of Main Sequence Stars in Galactic Globular Clusters I . CH and CN Bands in M 13 1

Spectra with a high signal-to-noise ratio of 50 stars which are just below the main sequence turnoff and are members of M13 are presented. They yield indices for the strength of the CH and the ultraviolet CN band. There is no evidence for a variation in the strength of either feature from star to star in this intermediate-metallicity galactic globular cluster, and thus no evidence for primordial variations in the abundance of C and N in M13. This supports the hypothesis that abundance variations found among the light elements in the evolved stars of M13 by Suntzeff (1981), and commonly seen on the giant and subgiant branches of globular clusters of comparable metallicity, are due primarily or entirely to mixing within a fraction of individual stars as they evolve. Subject headings: globular clusters: general — globular clusters: individual (M13) — stars: evolution