  We analize the structure of models with unbroken and spontaneously broken
U(1)_a x U(1)_b gauge symmetry. We show that the quantum corrections to the 2N
gauge charges, with N = #fermions + #scalars, can be absorbed in the
redefinition of three independent gauge couplings (g_a,g_b and g_ab). We
establish the (one-loop) conditions on the matter cotent for g_ab= = 0 (a value
usually assumed in the literature) and we show that in the minimal extensions
of the Standard Model with an extra U(1) symmetry the choice g_ab = 0 is not
stable under radiative corrections induced by the standard Higgs fields.
Moreover, g_ab = 0 to all orders seems to require an exact symmetry. The
spontaneous breaking of the gauge symmetry induces further mixing between the
two gauge bosons and introduces a fourth independent physical parameter. A
consequence of our analysis is that the usual tree-level description with only
three physical parameters (i.e., two gauge couplings and one gauge boson mixing
angle) is not in general a justified zero order limit of the treatment
including radiative corrections.
