New neutral gauge bosons and new heavy fermions in the light of the new LEP data

We derive limits on a class of new physics effects that are naturally present in grand unified theories based on extended gauge groups, and in particular in E_6 and SO(10) models. We concentrate on i) the effects of the mixing of a new neutral gauge bosons with the standard Z_0; ii) the effects of a mixing of the known fermions with new heavy states. We perform a global analysis including all the LEP data on the Z decay widths and asymmetries collected until 1993 the SLC measurement of the left-right asymmetry, the measurement of the W boson mass, various charged current constraints and the low energy neutral current experiments. We use a top mass value in the range announced by CDF. We derive limits on the Z_0--Z_1 mixing which are always \lsim 0.01, and are at the level of a few {\it per mille} if some specific model is assumed. Model dependent theoretical relations between the mixing and the mass of the new gauge boson in most cases require M_{Z'} > 1 TeV. Limits on light-heavy fermion mixings are also largely improved with respect to previous analyses, and are particularly relevant for a class of models that we discuss.