DIRICHLET BRANES AND RAMOND-RAMOND CHARGES

We show that D-branes, extended objects defined by mixed Dirichlet-Neu mann boundary conditions, break half the supersymmetries of the type I I superstring and carry a complete set of electric and magnetic Ramond -Ramond charges. The product of the electric and magnetic charges is a single Dirac unit, and the quantum of charge is that required by stri ng duality. This is strong evidence that D-branes are intrinsic to typ e II string theory and are the Ramond-Ramond sources needed for string duality. Also, we find in the IIa string a 9-form potential, which gi ves an effective cosmological constant.