Torons and D-brane bound states

Abstract We interpret instantons on a torus with twisted boundary conditions, in terms of bound states of branes. The interplay between the SU ( N ) and U (1) parts of the U ( N ) theory of N 4-branes allows the construction of a variety of bound states. The SU ( N ) and U (1) parts can contribute fractional amounts to the total instanton number which is integral. The geometry of non-self-intersecting 2-cycles in T 4 sheds some light on a number of properties of these solutions.