Oxygen isotope ratios have been measured in 21 ordinary N-type carbon stars and in five 13C-rich J-type stars. The ratios in the ordinary N-type stars are 550 ≤ 16O/17O ≤ 4100 and 700 ≤ 16O/18O ≤ 2400. These ratios are much higher than expected and are extremely difficult to explain in terms of present theories of stellar evolution. The J-type stars exhibit ratios 350 ≤ 16O/17O ≤ 850, 16O/18O ratios being very uncertain. Together with nitrogen abundances measured by Lambert et al., these ratios preclude the envelope burning (hot-bottom convection) hypothesis which has been proposed for these stars.
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Harris et al. (1987) studied this question.