ON the Interpretation of the Color-Magnitude Diagrams of M67 and NGC 188

A method for determining the helium content Y, for a given Z, of old galactic clusters is described. This method could be of considerable importance since it is independent of the distance modulus of the cluster and of color corrections such as those due to interstellar reddening and differential line blanketing. It is found that V is proportional to the width of the gap of the C-M diagram just above the main sequence In order to calibrate this effect, a number of theoretical evolutionary tracks was obtained from the main sequence to beyond the hydrogen-exhaustion phase for different chemical compositions. A sta- tistical analysis of the data of Eggen and Sandage was made which confirms the presence of such a gap in M67 and NGC 188 It shows that the gap width has a well-defined value equal to 0.167 ± 0.010 mag in M67. This last result yields V = 038 ± 0.02 at Z = 0 03 and V = 0.28 ± 0.02 at Z = 0.06. Higher V-values would be required if Z is as low as 001. The corresponding ages for M67 are about 3.5 X 1O~ years for V = 0.38, Z = 0.03, and 2 6 X 1O~ years for V = 0 28, Z = 0.06. In NGC 188, the gap, al- though present, does not appear to be fully developed, which leaves some uncertainty as to its chemical composition if this method is used