SELF-DUALITY OF BORN-INFELD ACTION AND DIRICHLET 3-BRANE OF TYPE IIB SUPERSTRING THEORY

D-brane actions depend on a world-volume abelian vector field and are described by Born-Infeld-type actions. We consider the vector field du ality transformations of these actions. Like the usual 2d scalar duali ty rotations of isometric string coordinates imply target space T-dual ity, this vector duality is intimately connected with SL(2, Z)-symmetr y of type IIB superstring theory. We find that in parallel with genera lised 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 pl ay a special role in type IIB theory and also suggests a possibility o f its 12-dimensional reformulation.