Upper panel: the quantity ξ (blue solid curve), the mean slope (dashed black curve) and the ratio of the time delays τ = H₀   Δt (dotted red curve) for the mass models κ_λ and that of the singular isothermal sphere, as a function of λ. Lower panel: the mass profiles of the model described by (6) (dashed black curve), two mass-sheet transformed ones (κ_0.8 – solid blue curve, and κ_1.19 – dotted red curve), and that of the corresponding SIS (thin black curve).