  We consider classes of T_6 orientifolds, where the orientifold projection
contains an inversion I_{9-p} on 9-p coordinates, transverse to a Dp-brane. In
absence of fluxes, the massless sector of these models corresponds to diverse
forms of N=4 supergravity, with six bulk vector multiplets coupled to N=4
Yang--Mills theory on the branes. They all differ in the choice of the duality
symmetry corresponding to different embeddings of SU(1,1)\times SO(6,6+n) in
Sp(24+2n,R), the latter being the full group of duality rotations. Hence, these
Lagrangians are not related by local field redefinitions. When fluxes are
turned on one can construct new gaugings of N=4 supergravity, where the twelve
bulk vectors gauge some nilpotent algebra which, in turn, depends on the choice
of fluxes.
