Constraints on top couplings in models with exotic quarks

The extension of the standard model with exotic quark singlets and doublets introduces large flavor changing neutral couplings between ordinary fermio ns. We derive inequalities which translate the precise determination of the diagonal Z couplings, in particular, at LEP, into stringent bounds on the off-diagonal ones. The resulting limits can be saturated in minimal extensi ons with one vector doublet or singlet. In this case, 23 and 6 single top e vents, respectively, are predicted at LEP2 for an integrated luminosity of 500 pb(-1) per experiment.