Symmetries of Heterotic String Effective Theory in Three and Two Dimensions

The four-dimensional bosonic effective action of the toroidally compactified heterotic string incorporating a dilaton, an axion and one $U(1)$ vector field is studied on curved space-time manifolds with one and two commuting Killing vectors. In the first case the theory is reduced to a three-dimensional sigma model possessing a symmetric pseudoriemannian target space isomorphic to the coset $SO(2,3)/(SO(3)\times SO(2))$. The ten-parameter group $SO(2,3)$ of target space isometries contains embedded both $S$ and $T$ classical duality symmetries of the heterotic string. With one more ignorable coordinate, the theory reduces to a two-dimensional chiral model built on the above coset, and therefore belongs to the class of completely integrable systems. This entails infinite-dimensional symmetries of the Geroch--Kinnersley--Chitre type. Purely dilatonic theory is shown to be two-dimensionally integrable only for two particular values of the dilaton coupling constant. In the static case (diagonal metrics) both theories essentially coincide; in this case the integrability property holds for all values of the dilaton coupling.