Maximum value of the ellipticity to avoid dumbbell-shaped mass distributions in the region close to the R_λ = ∞ contour as a function of . The solid line corresponds to ε_max obtained from the procedure in Sect. 3. The dashed line corresponds to the values of ε obtained from upper limits of the figure-of-merit representing the fractional deviation of the κ_ε contours with respect to an ellipse (Sect. 4).