  We present high-spatial resolution HST and adaptive optics observations, and
high-sensitivity ISO (ISOCAM & ISOPHOT) observations of a sample of X-ray
selected weak-line (WTTS) and post (PTTS) T Tauri stars located in the nearby
Chamaeleon T and Scorpius-Centaurus OB associations. HST/NICMOS and adaptive
optics observations aimed at identifying substellar companions at separations
>=30 A.U. from the primary stars. No such objects were found within 300 A.U. of
any of the target stars, and a number of faint objects at larger separations
can very likely be attributed to a population of background stars. ISOCAM
observations of 5 to 15 Myr old WTTS and PTTS in ScoCen reveal infrared
excesses which are clearly above photospheric levels, and which have a spectral
index intermediate between that of younger (1 to 5 Myr) T Tauri stars in Cha
and that of pure stellar photospheres. The difference in the spectral index of
the older PTTS in ScoCen compared to the younger classical and WTTS in Cha can
be attributed to a deficiency of smaller size (0.1 to 1mu) dust grains relative
to larger size (~5mu) dust grains in the disks of the PTTS. The lack of small
dust grains is either due to the environment (effect of nearby O stars and
supernova explosions) or due to disk evolution. If the latter is the case, it
would hint that circumstellar disks start to get dust depleted at an age
between 5 to 15 Myr. Dust depletion is very likely related to the build-up of
larger particles (ultimately rocks and planetesimals) and thus an indicator for
the onset of the period of planet formation.
