  We examine the potential of constraining possible nondecoupling effects of
heavy neutrinos and Higgs bosons at LEP and SLC that may show up in the
nonoblique part of the $Zl_il_j$ couplings. We analyze this type of new-physics
interactions within the context of low-energy scenarios motivated by unified
theories, such as the Standard Model (SM) with neutral isosinglets, the
left-right symmetric model, and the minimal supersymmetric SM. Our analysis
comprises a complete set of physical quantities based on the nonobservation of
flavour-violating $Z$-boson decays, lepton universality in the decays $Z\to
l\bar{l}$, and universality of lepton asymmetries at the $Z$ peak. It is found
that these quantities form a set of complementary observables and may hence
constrain the parameter space of the theories. Non-SM contributions of
new-physics interactions to $R_b=\Gamma(Z\to b\bar{b})/\Gamma(Z\to
\mbox{hadrons})$ are briefly discussed within these models.
