  In this paper we describe the heterotic dual of the type IIB theory
compactified to four dimensions on a toroidal orientifold in the presence of
fluxes. The type IIB background is most easily described in terms of an
M-theory compactification on a four-fold. The heterotic dual is obtained by
performing a series of U-dualities. We argue that these dualities preserve
supersymmetry and that the supergravity description is valid after performing
them. The heterotic string is compactified on a manifold that is no longer
Kahler and has torsion. These manifolds have to satisfy a number of
constraints, for example, existence of an holomorphic three-form, size limits,
torsional equations etc. We give an explicit form of the background and study
the constraints associated to them.
