Dilaton shift under duality and torsion of elliptic complex

We observe that the ratio of determinants of $2d$ Laplacians which appear in the duality transformation relating two sigma models with abelian isometries can be represented as a torsion of an elliptic (DeRham) complex. As a result, this ratio can be computed exactly and is given by the exponential of a local functional of $2d$ metric and target space metric. In this way the well known dilaton shift under duality is reproduced. We also present the exact computation of the determinant which appears in the duality transformation in the path integral.