Velocity shift ΔV as function of the ratio ξ =   (²⁵Mg+ ²⁶Mg)/²⁴Mg for the Mg i λ2852 Å (upper panel) and the Mg ii λ2796 Å (lower panel) lines. The curves are calculated for different ratios of the heavy isotopes, r =   ²⁶Mg/²⁵Mg (depicted over each curve). ΔV = 0 corresponds to the solar value of the Mg isotope ratios, ξ_⊙ = 0.2660 and r_⊙ ≈ 1. The horizontal dashed lines mark the mean values of ΔV_Mg   I and ΔV_Mg   II measured at z_abs  = 0.45207, whereas the dotted lines restrict the  ± 1σ uncertainty intervals (see text for details).