We present new 3 mm ATCA data of two Class I Young Stellar Objects in the Ophiucus star forming region : Elias29 and WL12 .
For our analysis we compare them with archival 1.1 mm SMA data .
In the ( u,v ) plane the two sources present a similar behavior : a nearly constant non-zero emission at long baselines , which suggests the presence of an unresolved component and an increase of the fluxes at short baselines , related to the presence of an extended envelope .
Our data analysis leads to unusually low values of the spectral index \alpha _ { 1.1 - 3 mm } , which may indicate that mm-sized dust grains have already formed both in the envelopes and in the disk-like structures at such early stages .
To explore the possible scenarios for the interpretation of the sources we perform a radiative transfer modeling using a Monte Carlo code , in order to take into account possible deviations from the Rayleigh-Jeans and optically thin regimes .
Comparison between the model outputs and the observations indicates that dust grains may form aggregates up to millimeter size already in the inner regions of the envelopes of Class I YSOs .
Moreover , we conclude that the embedded disk-like structures in our two Class Is are probably very compact , in particular in the case of WL12 , with outer radii down to tens of AU .