Anomalies and Nonsupersymmetric D-Branes

We revisit some aspects of D-brane theory from the point of view of anomalies. When the boundary condition on a worldsheet boson is flipped from Neumann to Dirichlet, worldsheet supersymmetry requires also reversing the sign of the boundary condition of the corresponding worldsheet fermion. This induces an anomaly which is a mod 2 anomaly in Type II superstring theory and a mod 8 anomaly in Type I superstring theory. The same anomaly also receives contributions from a sign in the sum over bulk spin structures (in Type IIA superstring theory), Chan-Paton factors of symplectic type (in Type I superstring theory), and Majorana fermions that propagate only on the worldsheet boundary. The need to cancel the anomaly accounts for many properties of supersymmetric and especially nonsupersymmetric D-branes in Type I and Type II superstring theory.