Charge and color breaking and D-terms in string theory

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.