Remarks on anomalous U(1) symmetries in string theory

Anomalous U(1) gauge symmetries appear frequently both in heterotic and type I/type II string theory. In the heterotic case we find at most a single anomalous U(1), while in open string theories several such symmetries can appear. We review the properties of the anomalous gauge bosons, the appearance of Fayet-Iliopoulos terms and the concept of heterotic-type I duality. Phenomenological applications of these symmetries might be different in the type I/type II case compared to the heterotic one.