CNO abundances of bright giants in the globular clusters M3 and M13

Spectra of the lambda 4300 CH and lambda 3883 CN bands have been obtained for a sample of red giants in M3 and M13 for which [O/Fe] abundances have been published in the literature. Measurements of the [C/Fe] and [N/Fe] abundances have been made by comparing the observed spectra with synthetic spectra calculated from model atmospheres. Giants in both clusters exhibit an anticorrelation between CN band strength and both [O/Fe] and [C/Fe] abundances, coupled with a CN-[N/Fe] correlation. The total [(C+N+O)/Fe] abundance is the same within the observational uncertainties among both CN-strong and CN-weak giants. These observations are in accord with the hypothesis that C-->N and O-->N processed material has been brought to the surface of the CN-strong cluster giants. (C) 1996 American Astronomical Society.