  Transition disks are protoplanetary disks with inner depleted dust cavities
and excellent candidates to investigate the dust evolution under the existence
of a pressure bump. A pressure bump at the outer edge of the cavity allows dust
grains from the outer regions to stop their rapid inward migration towards the
star and efficiently grow to millimetre sizes. Dynamical interactions with
planet(s) have been one of the most exciting theories to explain the clearing
of the inner disk. We look for evidence of the presence of millimetre dust
particles in transition disks by measuring their spectral index with new and
available photometric data. We investigate the influence of the size of the
dust depleted cavity on the disk integrated millimetre spectral index. We
present the 3mm photometric observations carried out with PdBI of four
transition disks: LkHa330, UXTauA, LRLL31, and LRLL67. We use available values
of their fluxes at 345GHz to calculate their spectral index, as well as the
spectral index for a sample of twenty transition disks. We compare the
observations with two kind of models. In the first set of models, we consider
coagulation and fragmentation of dust in a disk in which a cavity is formed by
a massive planet located at different positions. The second set of models
assumes disks with truncated inner parts at different radius and with power-law
dust size distributions, where the maximum size of grains is calculated
considering turbulence as the source of destructive collisions. We show that
the integrated spectral index is higher for transition disks than for regular
protoplanetary disks. For transition disks, the probability that the measured
spectral index is positively correlated with the cavity radius is 95%. High
angular resolution imaging of transition disks is needed to distinguish between
the dust trapping scenario and the truncated disk case.
