We discuss the sigma model description of a D-string bound to k D-five
branes in type I string theory. The effective theory is an (0,4) super
symmetric hyper-Kahler with torsion sigma model on the moduli space of
Sp(k) instantons on R-4. Upon toroidal compactification to five dimen
sions the model is related to the type II picture where the target spa
ce is a symmetric product of K3's. (C) 1998 Elsevier Science B.V.