A search for the decay Bs -> D*-+ pi+- is presented using a data sample corresponding to an integrated luminosity of 1.0 fb-1 of pp collisions collected by LHCb. This decay is expected to be mediated by a W-exchange diagram, with little contribution from rescattering processes, and therefore a measurement of the branching fraction will help to understand the mechanism behind related decays such as Bs -> pi+ pi- and Bs -> D Dbar. Systematic uncertainties are minimised by using B0 -> D*-+ pi+- as a normalisation channel. We find no evidence for a signal, and set an upper limit on the branching fraction of BR(Bs -> D*-+ pi+-) < 6.1 (7.8) x10^(-6) at 90% (95%) confidence level.
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