t → cWW and WW → tc + tc in extended models

Jenkins has pointed out that the process t → cW+W− is GIM suppressed in the standard model. In this note, we calculate the branching ratio for a wide range of models, in which the decay occurs at tree level through exchange of a scalar, fermion or vector. In the case of scalar exchange, a scalar mass between 2mW and 200 GeV leads to a resonant enhancement, giving a branching ratio as high as a few tenths of a percent. We then note that all of these models will also allow W+W− → tc + tc, and we calculate the single-top/single-charm production rate at the LHC. The rates are such that the background from single-top/single-bottom production will probably swamp the signal.