AdS3 backgrounds from 10D effective action of heterotic string theory

We present a method for calculating solutions and corresponding central charges for backgrounds with AdS3 and Sk factors in α'-exact fashion from the full tree-level low-energy effective action of heterotic string theory. Three examples are explicitly presented: AdS3xS3xT4, AdS3xS2xS1xT4, and AdS3xS3xS3xS1. Crucial property which enabled our analysis is vanishing of the Riemann tensor calculated from connection with ''σ-model torsion.'' We show the following: (i) Chern-Simons terms are the only source of α' corrections not only in BPS, but also in non-BPS cases, suggesting a possible extension of general method of Kraus and Larsen, (ii) our results are in agreement with some conjectures on the form of the part of tree-level Lagrangian not connected to a mixed Chern-Simons term by supersymmetry (and present in all supersymmetric string theories), (iii) new α'-exact result for central charges in AdS3xS3xS3xS1 geometry. As a tool we used our generalization of Sen's E-function formalism to AdSp with p>2, and paid special attention to proper definition of asymptotic charges.