Infra-red observations of CO band for globular and open clusters

IR photometric observations (J, H, K and 2.35-microns CO band) of 21 red giants in one globular and two old galactic clusters were carried out at the TIRGO telescope, on the assumption that the CNO content of globular clusters is related to the second-parameter problem. The clusters were selected to have the same metallicity but different stellar populations (M71 Population II, M67 and NGC2158 Population I). The observed CO distribution is analyzed. The relationship between the CO indices and CN absorption strengths as determined from DDO photometry is discussed; an anticorrelation CO/CN is found in M71 but not in M67. A simple explanation is proposed.