  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.
