Values of the spherical mass and concentration of a prolate NFW halo producing the same signal as that of a spherical NFW halo characterized by the S0 best-fit parameters. The different stars show the required (M_{200, 3D},c_{200, 3D}) as a function of the minor-to-major axis ratio a/c (starting from a/c = 1 in the top-right corner, and decreasing by 0.1). We consider here the case of a prolate halo (a = b<c) with a major axis aligned with the line of sight. The gray-shaded area and black curve indicate the relation of Duffy et al. 2008 (all haloes, z = 0.5).