Left panel: sketch of the ring model used to fit the transverse pv diagrams. The ring model is defined by five parameters illustrated in the figure and discussed in the text. Right panel: illustration of the fitting procedure performed on the transverse pv diagram at z = +0.7″. The black crosses show the trace determined according to the method described in text. The red line shows the result of the fit of this trace by an ellipse. The yellow ellipses show the distribution of solutions taking into account the estimated 1σ error on the five ring parameters. See text for more details.