members of M 13 from the photometric data, shown as black dots. The same procedures are performed to differentiate the likely member stars of M 15 and M 2 from the photometric sample.

We now proceed to select the stars that are likely members of the globular clusters from the available spectroscopic sample. This step begins by selection of the stars within the cluster tidal radii that pass the photometric criterion for membership, based on their location on the cluster CMDs according to the algorithm described above.

Figure 3 displays the cleaned CMD of M 13, overplotted with the likely members from the spectroscopic sample (shown as red circles). The same procedures are carried out to identify likely member stars of M 15 and M 2 from their spectroscopic data. Based on these cuts, at this stage of the analysis there are 296 (338) likely members for M 13, 124 (160) for M 15, and 21 (22) for M 2 identified. In the above, the first listed numbers indicate the stars with available estimates of \[Fe/H\] from the SSPP, while the quantities in parentheses represent the number of stars with available radial velocities (RVs). Additional cuts, based on the derived 