Abstract
We generalize the recent proposal that invariance under
T-duality leads to additional non-geometric fluxes required so that
superpotentials in type IIA and type IIB orientifolds match. We show
that invariance under type IIB S-duality requires the introduction of
a new set of fluxes leading to further superpotential terms. We find
new classes of 
= 1 supersymmetric Minkowski vacua based on
type IIB toroidal orientifolds in which not only dilaton and complex
moduli but also Kähler moduli are fixed. The chains of dualities
relating type II orientifolds to heterotic and M-theory
compactifications suggests the existence of yet further flux degrees
of freedom. Restricting to a particular type IIA/IIB or heterotic
compactification only some of these degrees of freedom have a simple
perturbative and/or geometric interpretation.
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