  In this work we study the production of color-neutral and singly-charged
heavy leptons at the proposed International Linear Collider. We use the optimal
observable technique to determine the statistical accuracy to which the
coupling of such fermions to the $Z$ gauge boson (vector, axial or chiral) can
be measured. We also consider a UV-complete model that contains these particles
as well as a dark matter candidate, and consider some observable effects
involving both; the correspondence to chargino production in supersymmetric
models with heavy sleptons is briefly discussed.
