Mixed boundary conditions and brane-string bound states

We consider open strings with mixed boundary conditions (a combination of Neumann and Dirichlet conditions at each end). We discuss how their end points show a D-p-brane with NS-NS charge, i.e. a bound state of a D-brane with a fundamental string. We show that these branes are BPS saturated. In the case of one-branes, we show that their mass densities are in agreement with IIb SUGRA which is Sl(2,Z) invariant. Using Chan-Paton factors, we extend our results to the case of bound states of n D-strings and m F-strings. These string theoretic results are also checked in the effective field theory limit. (C) 1998 Elsevier Science B.V.