THE SELF-DUAL TYPE-IIB SUPERTHREEBRANE

We find exact multi-threebrane soliton solutions of D = 10 type IIB supergravity which break half the supersymmetries, and saturate a Bogomol'nyi bound between the mass per unit volume and a conserved Noether charge. Since the type IIB five-index field strength is self-dual, however, the "electric" Noether charge and the "magnetic" topological charge are equal and we comment on the Dirac quantization rule for such threebrane "dyons". This self-dual threebrane represents a new point (d = 4, D = 10) on the brane-scan of supersymmetric extended objects. The gauge-fixed action on the worldvolume, corresponding to the zero modes of the soliton, is described by a four-dimensional Maxwell multiplet with N = 4 linearly realized and N = 4 non-linearly realized supersymmetries.