New photoelectric observations are given for 190 stars in and near M67. By using the properties of the (U - B, B - V) diagram it has been possible to separate the effects of interstellar reddening from those of line blanketing, and values of E(B - V) = +()N)6, E(U - B) = +(Y O4, A(B - V) = +()w()3, A(U - B) = +(YN)6 have been derived. The small value of the blanketing corrections, which indicates that the Fe/H abundance ratio is similar to that of the Sun and the Hyades cluster, shows that at least for some stars there has been very little eurichinent of heavy elements in the parent interstellar medium between the time of formation of M67 and the present. The color-magnitude diagram of the cluster shows the presence of a sharply defined gap on the rising branch of the evolved main sequence. This gap may represent a very rapid interlude on a gravitational time scale, between the depletion of the hydrogen in the stellar core and the firing of the hydrogen in a shell outside the core. Similar gaps are definitely present in NGC 2477 and NGC 752. The color-magnitude diagram of M67 also contains a horizontal branch populated by stars which are almost certainly cluster members on the basis of both the proper motions and the success with which the observed anomalies in their ultraviolet colors can be explained as a gravity effect. A comparison of evolutionary tracks, derived from a model by Hoyle, with the observed colormagnitude diagram gives good agreement except for a slight deviation near the main sequence caused by a more vertical rise to the evolved sequence than is predicted by theory. The distance modulus of M67, corrected for absorption, is m - M = 9".38 l 0"'2, obtained by using the method of the evolutionary deviation curve.
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Eggen et al. (1964) studied this question.