Comparison between the observed images at 0.87 mm, 1.3 mm, 2.7 mm and 7 mm (left panels, from top to bottom) and the images corresponding to the best fit model at each wavelength obtained by sampling the model visibilities at the same locations and with the same noise as for the observed data (middle panels). The images derived by the difference from the observed and model visibilities are shown on the right panels. At each wavelength the contours are the same in all three plots and correspond to −2σ and from 2σ to the peak value spaced by 2σ, where σ is the rms measured in the observed map (see also Fig. 1).