Time variation of the b velocity parameter for the CH⁺λ4232 (red), CN R(0) (blue) and CN R(1) (green) lines towards ζ Per between 2003 and 2011 (the time for CN R(1) values have been shifted by + 0.1 yr to avoid confusion with CN R(0) points). The instrumental broadening has been corrected assuming that the CH line width remained constant and equal to the value inferred from the Crane et al. spectrum. The weighted average of b(CN) R(0) and R(1) values are shown (blue line for R(0); green line for R(1)). Both CN lines are consistent with a constant value while CH⁺ shows some increase around 2005.