  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.
