  An interesting feature of some open superstring models in $D < 10$ is the
simultaneous presence, in the spectrum, of gauge fields and of a number of
antisymmetric tensor fields. In these cases the Green-Schwarz mechanism can
(and does) take a generalized form, resulting from the combined action of all
the antisymmetric tensors. These novelties are illustrated referring to some
simple rational models in six dimensions, and some of their implications for
the low-energy effective field theory are pointed out.
