Infra-red observations of stars in globular clusters

This paper presents a progress report on a programme of infra-red photoelectric photometry of the red giant sequences in some southern globular clusters. As measured in B and V these sequences are known to differ from cluster to cluster, at least one contributing factor being differential line-blanketing arising from variations in metal abundance. Observations in R and I should be substantially free of blanketing, and so enable us to eliminate this factor, clarifying the situation and making possible a more accurate transformation of the observations to the M_bol , log T_ett plane. The difficulty of making this transformation has been a major obstacle in comparing observations and theory for cluster giants.