D1/D5 systems in N=4 string theories Original Research Article

We propose CFT descriptions of the D1/D5 system in a class of freely acting Z2 orbifolds/orientifolds of type IIB theory, with sixteen unbroken supercharges. The CFTs describing D1/D5 systems involve N=(4,4) or N=(4,0) sigma models on (R3×S1×T4×(T4)N/SN)/Z2, where the action of Z2 is diagonal and its precise nature depends on the model. We also discuss D1(D5)-brane states carrying non-trivial Kaluza–Klein charges, which correspond to excitations of two-dimensional CFTs of the type (R3×S1×T4)N/SN⋉Z2N. The resulting multiplicities for two-charge bound states are shown to agree with the predictions of U-duality. We raise a puzzle concerning the multiplicities of three-charge systems, which is generically present in all vacuum configurations with sixteen unbroken supercharges studied so far, including the more familiar type IIB on K3 case: the constraints put on BPS counting formulae by U-duality are apparently in contradiction with any CFT interpretation. We argue that the presence of RR backgrounds appearing in the symmetric product CFT may provide a resolution of this puzzle. Finally, we show that the whole tower of D-instanton corrections to certain “BPS saturated couplings” in the low energy effective actions match with the corresponding one-loop threshold corrections on the dual fundamental string side.