radial subsamples. A large population of blue straggler stars (BS) is clearly visible, particularly in the inner part of the cluster where the background/foreground star contamination is low. In the intermediate zone, the BS population is better defined, and the sequence seems to reach brighter magnitudes. The BS sequence of the inner part appears to be broader in color than the sequence of the intermediate radial range. Part of this broadening can be attributed to the photometric errors that are larger in the inner region than in the intermediate one. The rest of the broadening is probably natural and could be connected to the two formation mechanisms of BS stars: the outer BS stars might mainly come from merger events, while the inner BS might be the final products of collisions (see Bailyn 1995).

We compared the distribution of the 95 BS with that of the 1669 sub-giant branch (SGB) stars selected in the same magnitude interval. In order to minimize the background star contamination along the SGB (very low indeed in the inner part of the cluster), we chose only the stars inside $\pm2.0\sigma$ (where $\sigma$ is the standard deviation in the mean color) from the mean position of 