center of mass. We used an iterative fitting algorithm (with the software suite “lensmod”, see Lehár et al. 1993, modified to operate in the image plane) that treats lensed images as points instead of extended objects. The $\chi^2$ goodness-of-fit measure for our models is a sum of squares of position differences (each weighted by the squared inverse of its standard error) for the images of the core. We also added a similarly-weighted contribution to $\chi^2$ from the coordinates of G. We found the “best-fit” lens model by direct search for minima of $\chi^2$ in parameter space, with the “amoeba” algorithm (see, e.g., Press et al. 1992). We analyzed in this way a series of different models (A $-$ D), each based on a different, but simple potential.

In our first model, A, we assumed a constant M/L$_I$ ratio for G (see Schneider et al. 1988), and used the elliptical isophotes for G (§3). We adjusted the M/L$_I$ ratio, represented by the radius of the Einstein ring of G, as well as $x_G$, $y_G$ and the mean position angle $\Psi$ of the light distribution of G. Thus, for model A we have 4 parameters and 8 constraints. Figure 7 shows the 