Effect of varying the Hα bandwidth in the correlation between Ca II and Hα coloured after activity level, measured as logR′_HK. Upper panel: stars with strong positive (ρ ≥ 0.5) or negative (ρ ≤ −0.5) correlation with the S_Hα06 index. Stars marked with thick lines are examples used in Figs. 4, 7, and A.2, and the only case of negative correlation with S_Hα06 (HD 206998). Lower panel: stars with weak or no correlation (−0.5 < ρ < 0.5) with the S_Hα06 index. In both panels, as an indication, we mark the boundaries between strong and no correlation with horizontal dotted lines at ρ values of 0.5 and −0.5.