Abstract: I will review the status of heterotic flux compactification with 8 and 4 supercharges.
The underlying geometry of these compactifications is a principal torus bundle over a K3 surface,
and T-duality in the torus fiber leads to equivalences between topologically distinct configurations.
In particular, I will argue that all such vacua that preserve 8 supercharges can be related to more conventional (flux free) compactifications.
These dualities lead to a rich interconnected web of stringy geometries with a fairly direct worldsheet CFT interpretation.
Orbifolds of such configurations can partially break supersymmetry and reduce the gauge group rank. On the other hand, there are four-dimensional configurations that cannot be T-dualized to configurations without flux; these configurations place strong constraints on the underlying K3 geometry.
If there is time, I will also discuss these equivalences from the dual M-theory point of view.