The paper deals with photometric observations of 82 stars on the upper giant branch of omega Centauri. The data show that omega Cen differs markedly from other globular clusters (studied in the infrared) in that, at a given V - K color, there is a wide spread in CO absorption in the 3 mag interval below the red giant tip and, at a given luminosity, V - K has a spread of up to 1 mag. Both ranges are much larger than those found for other clusters.
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Persson et al. (1980) studied this question.