  In the minimal extension of the standard electroweak theory with ultra
massive Majorana neutrinos, the process e- + e- -> mu- + mu- could be
observable, in sharp contrast with the reaction e- + e- -> W- + W- which is
entirely controlled by neutrinoless double beta decay $\beta \beta_{0 \nu}$
data. Our result provides the process background that must be confronted "new
physics" models which postulate doubly charged particles, such as the gauge
bilepton Y^{-} in the $SU(3)_{\rm c} \times SU(3)_{\rm L} \times U(1)$ model,
the left-right $SU(2)_{\rm L} \times SU(2)_{\rm R} \times U(1)_{\rm B-L}$ one,
and its supersymmetric version with doubly charged Higgs multiplets.
