The paper by Joseph Polchinski discusses the properties of Dirichlet-branes in type II superstring theory. Dirichlet-branes, defined by mixed Dirichlet-Neumann boundary conditions, break half of the supersymmetries of the type II superstring and carry a complete set of electric and magnetic Ramond-Ramond (RR) charges. The product of these charges is a single Dirac unit, and the quantum of charge aligns with the requirements of string duality. This evidence suggests that Dirichlet-branes are intrinsic to type II string theory and serve as the required RR sources for string duality. The paper also highlights the existence of a previously overlooked 9-form potential in the IIA string, which gives rise to an effective cosmological constant of undetermined magnitude. The 9-form potential couples to a 9-brane, but the specific nature of this coupling is not fully resolved. The paper concludes with implications for the consistency of orientifold compactifications and the potential impact of the 10-form potential on the cosmological constant.The paper by Joseph Polchinski discusses the properties of Dirichlet-branes in type II superstring theory. Dirichlet-branes, defined by mixed Dirichlet-Neumann boundary conditions, break half of the supersymmetries of the type II superstring and carry a complete set of electric and magnetic Ramond-Ramond (RR) charges. The product of these charges is a single Dirac unit, and the quantum of charge aligns with the requirements of string duality. This evidence suggests that Dirichlet-branes are intrinsic to type II string theory and serve as the required RR sources for string duality. The paper also highlights the existence of a previously overlooked 9-form potential in the IIA string, which gives rise to an effective cosmological constant of undetermined magnitude. The 9-form potential couples to a 9-brane, but the specific nature of this coupling is not fully resolved. The paper concludes with implications for the consistency of orientifold compactifications and the potential impact of the 10-form potential on the cosmological constant.