  We investigate the structure of the innermost region of three circumstellar
disks around pre-main sequence stars HD 142666, AS 205 N, and AS 205 S. We
determine the inner radii of the dust disks and, in particular, search for
transition objects where dust has been depleted and inner disk gaps have formed
at radii of a few tenths of AU up to several AU. We performed interferometric
observations with IOTA, AMBER, and MIDI in the infrared wavelength ranges
1.6-2.5um and 8-13um with projected baseline lengths between 25m and 102m. The
data analysis was based on radiative transfer simulations in 3D models of young
stellar objects (YSOs) to reproduce the spectral energy distribution and the
interferometric visibilities simultaneously. Accretion effects and disk gaps
could be considered in the modeling approach. Results from previous studies
restricted the parameter space. The objects of this study were spatially
resolved in the infrared wavelength range using the interferometers. Based on
these observations, a disk gap could be found for the source HD 142666 that
classifies it as transition object. There is a disk hole up to a radius of
R_in=0.30AU and a (dust-free) ring between 0.35AU and 0.80AU in the disk of HD
142666. The classification of AS 205 as a system of classical T Tauri stars
could be confirmed using the canonical model approach, i. e., there are no
hints of disk gaps in our observations.
