Carbon, nitrogen and oxygen in intermediate-mass supergiants : is oxygen underabundant ?

New CNO analyses of two southern Cepheids and four early F-type nonvariable supergiants are presented. Results for the new and previous CNO analyses of intermediate-mass supergiants of spectral type F, G, and K including the Cepheid variables are scrutinized for possible causes of the systematically low oxygen abundances found in these stars. A mild oxygen deficiency (0.3 dex relative to the solar abundance) is confirmed. After a comprehensive discussion of potential sources of error, it is concluded that the uncertainties arising from the abundance analysis, as well as the uncertainty surrounding the stars' initial composition, do not yet demand that the O deficiency be attributed exclusively to the dredge-up of ON-cycled material.