χ² contours of the combined fit of the 6.5 yr orbit corresponding to uncertainties of 1σ (blue contour) and the 90% confidence range (cyan contour) projected on planes consisting of various pairs of parameters. The best-fit solution is shown by the black dots. The magenta square (with error bars) in the third and fifth figure yields the parameters and their 1σ errors as determined from the fit of the radio light curve for s = 2 (see Sect. 5).