  Observations of extrasolar planets reveal that planets can be found in close
binary systems, where the semi-major axis of the binary orbit is less than 20
AU. The existence of these planets challenges planet formation theory, because
the strong gravitational perturbations due to the companion increase encounter
velocities between planetesimals and make it difficult for them to grow through
accreting collisions. We study planetesimal encounter velocities in binary
systems, where the planetesimals are embedded in a circumprimary gas disc that
is allowed to evolve under influence of the gravitational perturbations of the
companion star. We find that the encounter velocities between planetesimals of
different size strongly depend on the gas disc eccentricity. In all cases
studied, inclusion of the full gas dynamics increases the encounter velocity
compared to the case of a static, circular gas disc. Full numerical parameter
exploration is still impossible, but we derive analytical formulae to estimate
encounter velocities between bodies of different sizes given the gas disc
eccentricity. The gas dynamical evolution of a protoplanetary disc in a binary
system tends to make planetesimal accretion even more difficult than in a
static, axisymmetric gas disc.
