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 fermions. 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 extensions with one vector doublet or singlet. In this case, 23 and 6 single top events, respectively, are predicted at LEP2 for an integrated luminosity of 500 ${\mathrm pb}^{-1}$ per experiment.