  A gauge-independent approach to resonant transition amplitudes with
nonconserved external currents is presented, which is implemented by the pinch
technique. The analytic expressions derived with this method are $U(1)_{em}$
invariant, independent of the choice of the gauge-fixing parameter, and satisfy
a number of required theoretical properties, including unitarity. Although
special attention is paid to resonant scatterings involving the $\gamma WW$ and
$ZWW$ vertices in the minimal Standard Model, our approach can be extended to
the top quark or other unstable particles appearing in renormalizable models of
new physics.
