Millimeter flux density vs. spectral index for disk models for Elias29 (top panel) and WL12 (bottom panel). The black dots show our ATCA data. Each line represents the prediction of disk models with the same disk outer radius and dust opacity spectral index β, but increasing disk mass from left to right. For Elias29 the observed flux and spectral index are consistent with both a small (R ~ 15 AU) optically thick disk, in which case we cannot constrain the dust properties, or a relatively large (R ~ 50–200 AU) optically thin disk populated with cm-sized pebbles. For WL12 the match between the observed data and the models suggests a very small and optically thick disk with a 30 AU radius. It is not possible to determine whether the grains are in the μm- , mm- or cm-sized regime .