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A new color-magnitude diagram has been constructed from iris photometry of seven B and six V plates calibrated with Hawarden's photoelectric sequence supplemented by new photometry. Main-sequence fitting results in a giant branch clump that is significantly fainter than that for younger disk clusters or the horizontal branches of globular clusters. The implications of this are discussed in terms of the chemical abundance of the cluster. The age of Melotte 66 appears to be at least a billion years older that that of NGC 188, previously thought to be the oldest disk cluster.
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Anthony-Twarog et al. (1979) studied this question.
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