Context : Low-mass stars form with disks in which the coagulation of grains may eventually lead to the formation of planets .
It is not known when and where grain growth occurs , as models that explain the observations are often degenerate .
A way to break this degeneracy is to resolve the sources under study .

Aims : To find evidence for the existence of grains of millimetre sizes in disks around in T Tauri stars , implying grain growth .

Methods : The Australia Telescope Compact Array ( ATCA ) was used to observe 15 southern T Tauri stars , five in the constellation Lupus and ten in Chamaeleon , at 3.3 millimetre .
The five Lupus sources were also observed with the Submillimeter Array ( SMA ) at 1.4 millimetre .
Our new data are complemented with data from the literature to determine the slopes of the spectral energy distributions in the millimetre regime .

Results : Ten sources were detected at better than 3 \sigma with the ATCA , with \sigma \approx 1–2 mJy , and all sources that were observed with the SMA were detected at better than 15 \sigma , with \sigma \approx 4 mJy .
Six of the sources in our sample are resolved to physical radii of \sim 100 AU .
Assuming that the emission from such large disks is predominantly optically thin , the millimetre slope can be related directly to the opacity index .
For the other sources , the opacity indices are lower limits .
Four out of six resolved sources have opacity indices \lesssim 1 , indicating grain growth to millimetre sizes and larger .
The masses of the disks range from < 0.01 to 0.08 M _ { \odot } , which is comparable to the minimum mass solar nebula .
A tentative correlation is found between the millimetre slope and the strength and shape of the 10- \mu m silicate feature , indicating that grain growth occurs on similar ( short ) timescales in both the inner and outer disk .

Conclusions :