Anomalous U(1)'s in type I string vacua

We perform a systematic string computation of the masses of anomalous U (1) gauge bosons in four-dimensional orientifold vacua, and we study their localization properties in the internal (compactified) space. We find that N = 1 supersymmetric sectors yield four- dimensional contributions, localized in the whole six-dimensional internal space, while N = 2 sectors give contributions localized in four internal dimensions. As a result, the U(1) gauge fields can be much lighter than the string scale, so that when the latter is at the TeV, they can mediate new non-universal repulsive forces at submillimeter distances much stronger than gravity. We also point out that even U(1)'s which are free of four-dimensional anomalies may acquire non-zero masses as a consequence of six-dimensional anomalies. (23 refs).