  In this article 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 strings. 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.
