The C-12/C-13 ratio in stellar atmospheres. VIII - The very metal-deficient giant HD 122563

High-resolution Digicon spectra of the CH 4300-A band in the metal-deficient giant HD 122563 are analyzed for the atmospheric C-12/C-13 ratio. Nine (C-13)H lines are identified. A C-12/C-13 ratio of 5 + or - 2 is deduced. This low value shows that the present atmosphere has been seriously affected by internal mixing and processing. This extreme mixing is not predicted by stellar evolutionary calculations. It is suggested that the anomalous abundances of CNO may be explained on the assumption that modified products of He burning in the core or in a shell have been mixed to the surface.