Comparison between equivalent widths of 1719 and 1501 Å absorption indexes predicted by Starburst99 models (big squares color-coded by metallicity). We show in each diagram the EWs measured in five VANDELS stacks (big stars), obtained via the same procedure adopted for synthetic templates. The stacks used here were produced at different stellar mass bins (see Sect. 3.1 and Table 3), whose median stellar masses (in log₁₀ (M_*/M_⊙)) are highlighted in black above each point. The shaded gray area has the same meaning as in Fig. 2.