  D-brane actions depend on a world-volume abelian vector field and are
described by Born-Infeld-type actions. We consider the vector field duality
transformations of these actions. Like the usual 2d scalar duality rotations of
isometric string coordinates imply target space T-duality, this vector duality
is intimately connected with SL(2,Z)-symmetry of type IIB superstring theory.
We find that in parallel with generalised 4-dimensional Born-Infeld action, the
action of 3-brane of type IIB theory is SL(2,Z) self-dual. This indicates that
3-brane should play a special role in type IIB theory and also suggests a
possibility of its 12-dimensional reformulation.
