Top panel. m_F160W vs. m_F110W − m_F160W CMD for NGC 6397 stars derived from IR/WFC3 data. Bottom panels. Color width of the M-dwarf sequence, W_F110W,F160W, as a function of the logarithm of the cluster mass (left) and the internal oxygen variation (right). The black dot represents the open cluster NGC 6791, NGC 6397 is shown in azure, and the remaining GCs are displayed with white dots (Dondoglio et al. 2022). The Spearman’s rank correlation coefficients are quoted in each panel.