Compactification with flux on K3 and tori

We study compactifications of type-IIB string theory on a K3 × T2/Z2 orientifold in the presence of RR and NS flux. We find the most general supersymmetry preserving, Poincare invariant, vacua in this model. All the complex structure moduli and some of the Kahler moduli are stabilised in these vacua. We obtain in an explicit fashion the restrictions imposed by supersymmetry on the flux, and the values of the fixed moduli. Some T-duals and Heterotic duals are also discussed, these are non-Calabi-Yau spaces. A superpotential is constructed describing these duals. Finally, we consider some susy breaking vacua where the flux gives rise to large extra dimensions with vanishing tree-level cosmological constant