We study duality transformations for two-dimensional sigma models with
abelian chiral isometries and prove that generic such transformations are
equivalent to integrated marginal perturbations by bilinears in the chiral
currents, thus confirming a recent conjecture by Hassan and Sen formulated in
the context of Wess-Zumino-Witten models. Specific duality transformations
instead give rise to coset models plus free bosons.Comment: 15 page