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 local functional of 2d metric and target space metric. In this way the well-known dilation shift under duality is reproduced. We also present the exact computation of the determinant which appears in the duality transformation in the path integral.