HIGH-RESOLUTION SPECTRA OF STARS IN GLOBULAR-CLUSTERS .6. OXYGEN-DEFICIENT RED GIANT STARS IN M13

From high-resolution, high signal-to-noise spectra we have determined abundances of carbon, nitrogen, and oxygen and the C-12/C-13 ratio for five red giants in M13, including star II-67, which has previously been reported to be deficient in oxygen. Three of the five stars exhibit substantial oxygen deficiencies; [O/Fe] values range from +0.5 to less-than-or-equal-to -0.3. The sum of the CNO nuclides is the same for all stars, which we interpret as evidence that mixing of CNO-cycled material into the envelope is the cause of the variations in oxygen abundance.