  Context.Transition disks are believed to be the final stages of
protoplanetary disks, during which a forming planetary system or
photoevaporation processes open a gap in the inner disk, drastically changing
the disk structure. From theoretical arguments it is expected that dust growth,
fragmentation and radial drift are strongly influenced by gas disk structure,
and pressure bumps in disks have been suggested as key features that may allow
grains to converge and grow efficiently.
  Aims. We want to study how the presence of a large planet in a disk
influences the growth and radial distribution of dust grains, and how
observable properties are linked to the mass of the planet.
  Methods. We combine two-dimensional hydrodynamical disk simulations of
disk-planet interactions with state-of-the-art coagulation/fragmentation models
to simulate the evolution of dust in a disk which has a gap created by a
massive planet. We compute images at different wavelengths and illustrate our
results using the example of the transition disk LkCa15.
  Results. The gap opened by a planet and the long-range interaction between
the planet and the outer disk create a single large pressure bump outside the
planetary orbit. Millimeter-sized particles form and accumulate at the pressure
maximum and naturally produce ring-shaped sub-millimeter emission that is
long-lived because radial drift no longer depletes the large grain population
of the disk. For large planet masses around 9 $M_{\mathrm{Jup}}$, the pressure
maximum and, therefore, the ring of millimeter particles is located at
distances that can be more than twice the star-planet separation, creating a
large spatial separation between the gas inner edge of the outer disk and the
peak millimeter emission. Smaller grains do get closer to the gap and we
predict how the surface brightness varies at different wavelengths.
