  We discuss R-symmetry in locally supersymmetric $N=2$ gauge theories coupled
to hypermultiplets, which can be viewed as effective theories of heterotic
string models. In this type of supergravities a suitable R-symmetry exists and
can be used to topologically twist the theory. The vector multiplet of the
dilaton-axion field has a different R-charge assignment with respect to the
other vector multiplets.
