  We study the implications of target-space duality symmetries for low-energy
effective actions of various four-dimensional string theories. In the heterotic
case such symmetries can be incorporated in simple orbifold examples. At
present a similar statement cannot be made about the simplest type IIB
orientifolds due to an obstruction at the level of gravitational anomalies.
This fact confirms previous doubts concerning a conjectured heterotic-type IIB
orientifold duality and shows that target-space symmetries can be a powerful
tool in studying relations between various string theories at the level of the
effective low-energy action. Contraints on effective Lagrangians from these
symmetries are discussed in detail. In particular, we consider ways of
extending T-duality to include additional corrections to the Kaehler potential
in heterotic string models with N=2 subsectors.
