  Jenkins has pointed out that the process $t\to cW^+W^-$ 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 $2m_W$ 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 $W^+W^-\to \bar{t}c+t\bar{c}$, and we calculate the
single-top/single-charm production rate at the LHC. The rates aren't negligibly
small, but the background from single-top/single-bottom production will
probably swamp the signal.
