  It is shown that the super Higgs mechanism that occurs in a wide class of
models with vanishing cosmological constant (at the classical level) is
obtained by the gauging of a flat group which must be an electric subgroup of
the duality group. If the residual massive gravitinos which occur in the
partial supersymmetry breaking are BPS saturated, then the flat group is non
abelian. This is so for all the models obtained by a Scherk-Schwarz
supersymmetry breaking mechanism. If gravitinos occur in long multiplets, then
the flat groups may be abelian. This is the case of supersymmetry breaking by
string compactifications on an orientifold T^6/Z_2 with non trivial brane
fluxes.
