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