Equivalent width of the CH⁺λ4232 line (upper panel) in ζ Per spectra between 2003 and 2011 (same symbols as in Fig. 1) together with the corresponding b value measured from high resolution McD data (lower panel). Thick error bars indicate measurements used to assess the significance of a maximum in W and b around (“in” values; see text). The b values have been corrected for instrumental broadening (see text). Note that both W and b display a maximum around 2005. In each panel, the horizontal line indicates the weighted average computed outside the time interval over which W was larger.