Same as in Fig. 2, but for the two Fe iλ2719 and Fe iλ3021 lines seen at z_abs = 0.4521 towards HE 0001−2340. For C_f = 1, the range allowed for r′ is very small (red horizontal lines). The behaviour of r′ is shown for τ₀(Fe iλ3021) = 1, ≪1 and ≫1 (thin blue lines); r′ depends very little on line opacities. The observed ratio, r′ = 0.32 ± 0.07 (plain black horizontal line, with 1σ uncertainties shown as dotted lines) is clearly outside the allowed range. It implies a value C_f ≈ 0.33 (thick blue cross), compatible with C_f = C_f,min thus C_elr = 0 (thick black vertical line), irrespective of τ₀(Fe iλ3021).