U(1) charges and moduli in the D1-D5 system

The decoupling limit of the D1-D5 system compactified on T4xS1 has a rich spectrum of U(1) charged excitations. Even though these states are not BPS in the limit, BPS considerations determine the mass and the semiclassical entropy for a given charge vector. The dependence of the mass formula on the pactification moduli situates the symmetric orbifold SymN(T4)xT4 conformal field theory in the moduli space. A detailed analysis of the global identifications of the moduli space yields a picture of multiple weak-coupling limits - one for each factorization of N into D1 and D5 charges d1 and d5=N/d1 - joined through regions of strong coupling in the CFT moduli space. (author)