D-BRANES, MODULI, AND SUPERSYMMETRY

We study toroidal compactifications of type II string theory with D-br anes and nontrivial antisymmetric tensor moduli and show that turning on these fields modifies the supersymmetry projections imposed by D-br anes. These modifications are seen to be necessary for the consistency of T duality. We also show the existence of unusual BPS configuration s of branes at angles that are supersymmetric because of conspiracies between moduli fields. Analysis of the problem from the point of view of the effective field theory of massless modes shows that the presenc e of a two-form background must modify the realization of supersymmetr y on the brane. In particular, the appropriate supersymmetry variation of the physical gaugino vanishes in any constant field strength backg round. These considerations are relevant for the E-7(7)-symmetric coun ting of states of four-dimensional black holes in type II string theor y compactified on T-6.