AdS3 backgrounds from 10D effective action of heterotic string theory

We present a method for calculating solutions and corresponding central charges for backgrounds with AdS(3) and S-k factors in alpha'-exact fashion from the full tree-level low-energy effective action of heterotic string theory. Three examples are explicitly presented: AdS(3) x S-3 x T-4, AdS(3) x S-2 x S-1 x T-4, and AdS(3) x S-3 x S-3 x S-1. Crucial property which enabled our analysis is vanishing of the Riemann tensor calculated from connection with "sigma-model torsion.'' We show the following: (i) Chern-Simons terms are the only source of alpha' 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 alpha'-exact result for central charges in AdS(3) x S-3 x S-3 x S-1 geometry. As a tool we used our generalization of Sen's epsilon-function formalism to AdS(p) with p > 2, and paid special attention to proper definition of asymptotic charges.