Signals of new gauge bosons at future e+e- colliders

We analyze possible indirect signals of additional neutral gauge bosons at future e+e- colliders, concentrating on SU(2)LxU(1)YxU(1)sub(Y') and SU(2)LxSU(2)RxU(1) effective theories. We develop a simple formalism to describe these effects and make a careful study of radiative corrections, in particular initial state radiation, which will be shown to have important implications. To make realistic estimates of the sensitivity to the new gauge boson effects, we use a model detector for e+e- annihilation at a center of mass energy of 500 GeV. Using a number of selected physical observables we then show that masses considerably higher than the total energy [up to a factor of 6] can be probed and that distinction between various theoretical models is possible. (orig.)