Target-space duality in heterotic and type I effective Lagrangians

We study the implications of target-space duality symmetries for low-energy effective actions of various four-dimensional string theories. In the hete rotic 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 po werful tool in studying relations between various string theories at the le vel 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 Kahler potential in heterotic string models with N = 2 subsectors. (C) 2000 Elsev ier Science B.V. All rights reserved.