Anomalous giant branch of NGC 188

The giant branch of NGC 188 has been analyzed relative to that of M67, and it is shown that a significant difference exists between the two clusters in that the giant branch of NGC 188 is intrinsically redder than the giant branch of M67. It is concluded that the most probable cause of the difference and of the observed scatter in the more luminous region of the giant branch in NGC 188 is the combined effect of mass loss, both early on in post-main-sequence evolution and at the tip of the giant branch, and a difference in chemical composition between the two clusters; NGC 188 has lower helium and/or higher CNO abundance than M67. The effect of this on the ages and distance moduli of the clusters is discussed