ZZ' mixing and radiative corrections at LEP I

We present a method for a common treatment of Z' exchange, QED corrections, and weak loops in e^+ e^- annihilation. QED corrections are taken into account by convoluting a hard-scattering cross section containing \gamma,\ Z, and Z' exchange. Weak corrections and ZZ' mixing are treated simultaneously by a generalization of weak form factors. Using the properly extended Standard Model program for the Z line shape, ZFITTER, we perform and compare two different analyses of the 1990 LEP~I data in terms of theories based on the E_6-group and in terms of LR-symmetric models. From the LEP~I data alone, the ZZ' mixing angle may be limited to |\theta_M| \le 0.01 and the Z' mass to M_2 > 118--148 GeV, depending on the model (95\%~CL).