uvbyHβ photoelectric and CCD photometry of the old, open southern cluster IC 4651 is presented and analyzed. The combined data lead to the following cluster characteristics: E(b-y) = 0.064±0.006, [Fe/H] = +0.23±0.02, (m-M)0 = 9.83±0.04, and an age on the system of VandenBerg (1985) of 2.4±0.2×109yr. The cluster color-magnitude diagram exhibits some scatter, all of which can be explained by binaries, non-members, and a blueward hook due to the hydrogen-exhaustion phase. The cluster turnoff shows marginal evidence for the presence of bimodality found in NGC 752, a younger and more metal-poor cluster. It is speculated that the bimodality is the result of nonstandard evolution near the Böhm-Vitense gap.
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Anthony-Twarog et al. (1987) studied this question.
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