Variations of the Si ii 6347 Å  line. Upper panel: grey-scale representation. Bottom panel: typical profiles (Ondřejov data, R ~ 12 500). The sharp vertical line denotes the laboratory wavelength λ₀. The position of λ₀ in the frame connected with the HD 50138 system is also plotted together with its error bar. The line shift is determined by the RV of the system rv_sys = 40 ± 4 km s^-1 based on the RVs of the [O i] λλ 6300, 6364 Å  lines.