added an external shear parameterized by $\gamma_e$ as in WMS95, measured in units of the critical surface mass density $\Sigma_c$. Model D was the same as model C, except that we set $P=2$, which corresponds to a point mass. With both of these models, we obtained improved fits, with final $\chi^2/DOF$ of $\sim 5.4$ and $\sim 4.5$, respectively (see Figure 7). Table 4 shows the parameter values for models $B-D$. The errors quoted in the table are the changes in the parameters that separately yield an increase of $\chi^2/DOF$ of 1. These errors do not account for the systematic effects that cause $\Delta\chi^2 > 1$.

The 15 GHz flux ratios of the images of the core were recently shown to be consistent with no fluctuations, at the $\sim 3$% rms level over a period of 6 months (Moore & Hewitt 1996, hereafter MH96). We re-estimated the parameter values in each of our models A-D, after including the mean KH93 flux ratios as constraints, with $\sim 3$% errors. We found that $\chi^2/DOF$ for the model A$-$D fits worsened by 5, 12, 40 and 40%, respectively. These new parameter values led to predicted image flux ratios that contributed insignificantly to $\chi^2$, whereas 