  In four dimensional superstring models, the gauge group normally contains
extra U(1)s that are broken at a high energy scale. We show that the presence
of the extra U(1)s is crucial for the phenomenological viability of string
scenarios, since the contribution to the scalar masses from the D-terms can
lift the unbounded from below directions that usually appear when the gauge
group at high energies is just the Standard Model group. In particular, we show
that the dilaton dominated scenario can be allowed in large regions of the
parameter space if there exists an anomalous U(1) in the theory, and the
charges of the particles are the appropriate ones. We also analyze the
parameter space of some explicit string constructions, imposing a correct
phenomenology and the absence of dangerous charge and color breaking minima or
unbounded from below directions.
