Visibility amplitudes vs. u–v distances for Elias29 at both 1.1 and 3 mm. The orange line is our best fit of the data, obtained with a 2 M_⊙ envelope, where grains have already grown up to mm size. The green line represents a model obtained with a 1.1 M_⊙ envelope, with grains up to the micrometer size, in order to reproduce the 1.1 mm observations. It fails, however, to reproduce the 3 mm data. This shows that only the presence of already grown grains can give the dust an opacity able to reproduce the dual-frequency observations.