Notes. The two upper models were optimized by exploiting the eight observed positions of the spectroscopically confirmed background sources A and B, while the four bottom models include also the observed positions of the two multiple images of source C, without a spectroscopic redshift measurement. For each model, we show the best-fit values of the parameters, the number of degrees of freedom (d.o.f.), the minimum chi-square () value, and the resulting BIC, AICc and rms values. The values of the x and y coordinates are referred to the center of light of the main elliptical lens galaxy, i.e., to its brightest pixel. The position angle θ is measured counterclockwise from x. Values in square brakets are kept fixed.