  The Kepler mission has discovered about a dozen circumbinary planetary
systems, all containing planets on ~ 1 AU orbits. We place bounds on the
locations in the circumbinary protoplanetary disk, where these planets could
have formed through collisional agglomeration starting from small (km-sized or
less) planetesimals. We first present a model of secular planetesimal dynamics
that accounts for the (1) perturbation due to the eccentric precessing binary,
as well as the (2) gravity and (3) gas drag from a precessing eccentric disk.
Their simultaneous action leads to rich dynamics, with (multiple) secular
resonances emerging in the disk. We derive analytic results for size-dependent
planetesimal eccentricity, and demonstrate the key role of the disk gravity for
circumbinary dynamics. We then combine these results with a simple model for
collisional outcomes and find that in systems like Kepler 16, planetesimal
growth starting with 10-100 m planetesimals is possible outside a few AU. The
exact location exterior to which this happens is sensitive to disk
eccentricity, density and precession rate, as well as to the size of the first
generation of planetesimals. Strong perturbations from the binary in the inner
part of the disk, combined with a secular resonance at a few AU inhibit the
growth of km-sized planetesimals within 2 - 4 AU of the binary. In situ
planetesimal growth in the Kepler circumbinary systems is possible only
starting from large (few-km-sized) bodies in a low-mass disk with surface
density less than 500 g/cm^2 at 1 AU.
