  We study planetesimal evolution in circumbinary disks, focusing on the three
systems Kepler 16, 34 and 35 where planets have been discovered recently. We
show that for circumbinary planetesimals, in addition to secular forcing,
eccentricities evolve on a dynamical timescale, which leads to orbital
crossings even in the presence of gas drag. This makes the current locations of
the circumbinary Kepler planets hostile to planetesimal accretion. We then
present results from simulations including planetesimal formation and dust
accretion, and show that even in the most favourable case of 100% efficient
dust accretion, in situ growth starting from planetesimals smaller than ~10 km
is difficult for Kepler 16b, Kepler 34b and Kepler 35b. These planets were
likely assembled further out in the disk, and migrated inward to their current
location.
