  The mixing of the known quarks with new heavy singlets can modify
significantly some observables with respect to the Standard Model predictions.
We analyse the range of deviations permitted by the constraints from precision
electroweak data and flavour-changing neutral processes at low energies. We
study top charged current and neutral current couplings, which will be directly
tested at top factories, like LHC and TESLA. We discuss some examples of
observables from K and B physics, as the branching ratio of K_L -> pi^0 nu nu,
the B^0_s mass difference or the time-dependent CP asymmetry in B_s^0 -> D_s^+
D_s^-, which can also show large new effects.
