  We analyse different N=4 supergravities coupled to six vector multiplets
corresponding to low-energy descriptions of the bulk sector of T6/Z2
orientifolds with p-brane in IIB (p odd) and in IIA (p even) superstrings. When
fluxes are turned on, a gauging emerges corresponding to some non-semisimple
Lie algebra related to nilpotent algebras N_p inside so(6,6), with dimension 15
+ (p-3)(9-p). The non-metric axions have Stueckelberg couplings that induce a
spontaneous breaking of gauge symmetries. In four cases the gauge algebra is
non-abelian with a non-commutative structure of the compactification torus, due
to fluxes of NS-NS and R-R forms.
