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 obtai ned for a sample of red giants in M3 and M13 for which [O/Fe] abundanc es have been published in the literature. Measurements of the [C/Fe] a nd [N/Fe] abundances have been made by comparing the observed spectra with synthetic spectra calculated from model atmospheres. Giants in bo th clusters exhibit an anticorrelation between CN band strength and bo th [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 observat ions 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 gian ts. (C) 1996 American Astronomical Society.