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Grains in disks around young stars grow from interstellar submicron sizes toplanetesimals over the course of several Myr. Thermal emission of large grainsor pebbles can be best observed at cm wavelengths. However, other emissionmechanisms can contribute. We aim to determine the mechanisms of cm emissionfor 3 T Tauri stars. WW Cha and RU Lup were recently found to have grain growthat least up to mm sizes in their circumstellar disks. CS Cha has similarindications for grain growth in its circumbinary disk. The T Tauri stars WW Chaand RU Lup were monitored over several years at mm and cm wavelengths, usingATCA. The new ATCA 7 mm system was also used to observe CS Cha. WW Cha wasdetected on several occasions at 7 and 16 mm. We obtained one detection of WWCha at 3 cm and upper limits only at 6 cm. The emission at 16 mm was stableover days, months and years, but the emission at 3 cm is found to be variable.RU Lup was detected at 7 mm. It was observed at 16 mm 3 times and at 3 and 6 cm4 times and found to be variable in all 3 wavebands. CS Cha was detected at 7mm, but the S/N was too low to resolve the gap in the circumbinary disk. Theemission at 3, 7 and 16 mm for WW Cha is well explained by thermal emissionfrom mm and cm-sized pebbles. The cm spectral index is consistent with theemission from an optically-thick ionised wind, but the high variability of thecm emission points to a non-thermal contribution. The SEDs of RU Lup and CS Chafrom 1 to 7 mm are consistent with thermal emission from mm-sized grains. Thevariability of the longer-wavelength emission for RU Lup and the negativespectral index suggest non-thermal emission.

Large grains in disks around young stars: ATCA observations of WW Cha, RU Lup, and CS Cha