  We generalize the inclusion of the imaginary parts of the fermionic one-loop
corrections for processes with unstable vector bosons to the case of massive
external fermions and non conservation of weak currents. We study the effect of
initial and final state fermion masses in single W production in connection
with the gauge-invariant treatment of the finite-width effects of W and Z
bosons, giving numerical comparisons of different gauge-invariance-preserving
schemes in the energy range of LEP2 and LC for e+e- -> e- v u d. We do not find
significant differences between the results obtained in the imaginary part
fermion loop scheme and in other exactly gauge preserving methods.
