  We analyse the allowed range of values of chi, both in the Standard Model and
in models with New Physics, pointing out that a relatively large value of chi,
e.g. of order lambda, is only possible in models where the unitarity of the 3x3
Cabibbo-Kobayashi-Maskawa matrix is violated through the introduction of extra
Q=2/3 quarks. We study the interesting case where the extra quark is an
isosinglet, determining the allowed range for chi and the effect of a large chi
on various low-energy observables, such as CP asymmetries in B meson decays. We
also discuss the correlated effects which would be observable at high energy
colliders, like decays t -> cZ, modifications of the cross section and
forward-backward asymmetry in e+ e- -> ttbar and the direct production of a new
quark.
