  The Born--Infeld theory of a D3-brane intersecting with(p,q) strings is
reconsidered. From the assumption that the electromagnetic fields are those of
a dyon, and using the kappa invariance of the action, the explicit scalar field
and its charge are derived. Considering perturbations orthogonal to both
branes, the SL(2,Z)-invariant S-matrix is obtained. Owing to the selfduality of
the brane the latter can be evaluated explicitly in both high and low energy
regions.
