  If new leptons exist close to the electroweak scale, they can be produced in
pairs at LHC through standard or new interactions. We study the production of
heavy lepton pairs in SM extensions with: (i) a Majorana or Dirac lepton
triplet, as those appearing in type-III seesaw; (ii) a lepton isodoublet
(N,E)_{L,R}; (iii) a charged isosinglet E_{L,R}; (iv) a Majorana or Dirac
neutrino singlet N and an additional Z' gauge boson. It is shown that the
trilepton final state l+-l+-l-+, which has a small SM background, constitutes
the golden channel for heavy neutrino searches, being very sensitive to
Majorana or Dirac neutrinos in triplet, doublet or singlet SU(2)_L
representations. For higher luminosities, signals in this final state can also
distinguish lepton triplets from doublets and singlets. The Majorana or Dirac
nature of the heavy neutrinos is revealed by the presence or not of like-sign
dilepton l+-l+- signals without significant missing energy. Notably, l+-l+-
signals but with large missing energy are characteristic of Dirac triplets,
distinguishing them from the other two models with a heavy Dirac neutrino.
Further discrimination is achieved with the analysis of the clean l+l+l-l-
final state.
