Fit of the ring model: left panel shows the maser positions in circles and the fitted ellipse, with the  +  sign designating the centre of the ellipse. The star indicates the position of the 7 mm continuum object identified by Curiel et al. (2006). Right panel shows the measured velocities (circles) as a function of the position angle (PA) in our ring model. The solid line shows the fit to a radial velocity model and the dashed line shows the result of a Keplerian velocity model.