D-brane bound states and the generalised ADHM construction Original Research Article

We discuss the sigma model description of a D-string bound to k D-fivebranes in type I string theory. The effective theory is an (0, 4) supersymmetric hyper-Kähler with torsion sigma model on the moduli space of Sp(k) instantons on R4. Upon toroidal compactification to five dimensions the model is related to the type II picture where the target space is a symmetric product of K3ʹs.