those calculated by Dotter et al.(2015, see their Figures 9 and 12) who used ATLAS12 and similar chemical compositions for populations A and C.

The reason why these isochrones provide a better fit with the observed CMD is that the split MS is a natural consequence of the fact that population-A stars have higher oxygen abundance than population-C stars. Indeed, the F160W band is heavily affected by absorption from H$_{2}$O molecules, which is stronger in MS-A stars than in MS-C stars, while the F110W filter is almost unaffected by the oxygen abundance. As a consequence, MS-A stars have fainter F160W magnitudes and bluer F110W$-$F160W colors than MS-C stars with the same luminosity. MS-B stars have intermediate properties.

![image](f3a.ps){height="7.5cm"} ![image](f3b.ps){height="7.5cm"}

The three stellar populations of NGC6752
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![image](f4.ps){width="11.0cm"}

To derive the relative numbers of stars in each MS, we followed the procedure illustrated in Fig. \[fig:pratio\]. The left panel of Fig. \[fig:pratio\] is a zoom of Fig. \[fig:cmd\] around the region of the NIR CMD where the triple sequence is clearly distinguishable and the blue and red lines overimposed on the CMD are the fiducials of MS-A and MS-C, respectively. To derive the red (blue) fiducial we applied a procedure based 