Comparison of ΣW vs. V − V_HB using both our and R97 data. The R97 EWs were corrected using the factor 1.117 found in Sect. 5.2. The solid lines are linear fits with fixed slope a =  −0.627. When our data are plotted, the fitted lines to R97 data are also shown as dashed lines. A radial velocity cut was imposed to R97a data to select cluster members: ΔRV > 38   km   s^-1 and ΔRV < 52.75   km   s^-1 define cluster members for NGC 6528 and NGC 6553, respectively.