VIRTUAL EFFECTS OF HEAVY CHIRAL FERMIONS AT E(-) COLLIDERS()E()

We derive the low-energy electroweak effective Lagrangian for the case of additional heavy, unmixed, sequential fermions. Present LEP1 data still allow the presence of new fermionic doublets (quarks and/or lept ons), with masses greater than M(Z)/2, provided that these multiplets are sufficiently degenerate. Deviations of the effective Lagrangian pr edictions from full one-loop computation are sizeable only for fermion masses close to the threshold M(Z)/2. We analyse the contribution of new heavy sequential fermions to the e(+)e(-) --> W+W- cross-section a t the energies of LEP2 and Next Linear Collider (NLC). The signals com ing from an additional doublet will be out of the LEP2 observability l evel. More interesting is the case of NLC where the simultaneous prese nce of the kinematic enhancement factor p(2)/m(W)(2) and of the unitar ity delay effect gives deviations from the Standard Model of the order of 10-50 per cent, for a wide range of new fermion masses.