Young stars in the old galactic cluster NGC 188?

First, the age of the oldest known galactic cluster, according to recently published determinations, is briefly discussed. The now definitely established membership of four W UMa type contact binaries in this cluster is difficult to understand if the age of these stars is that of the cluster. It appears therefore that these binaries are much younger and that several episodes of star formation took place in NGC 188. This conclusion is reached after a new study of the mean density of the four contact binaries and a critical discussion of the chemical composition and the mixing length parameter.