Modeling results and observations of the dust surface density, comparing δ_dustcav ranging between 10^-2 and 10^-8 as indicated in the right panel. The observations are plotted in black. Left: spectral energy distribution. Middle left: amplitude of the 690 GHz continuum visibility for the deprojected baselines. The null line is indicated with a dashed line. Middle right: normalized intensity cuts through the major (bottom) and minor (top) axis of the 690 GHz continuum image. The model images are convolved with the same beam as the ALMA observations. Right: dust surface density profile. Indicated are the δ_dust, the drop in density to fit the inner disk through the near-infrared emission, and δ_dustcav, the minimum drop in dust density inside the cavity to fit the observations. In SR21 (top panel) the region inside 7 AU is assumed to be empty because of additional constraints (see main text). In all other disks δ_dustcav is found to be at most 10^-4. All other disk parameters are as listed in Table 4.