As can be seen from Table 2, for t 3 (f N α ) = t 3 (f K α ) very stringent constraints can be derived for F ij L,R , (i = j) which strongly limits the mixing of ordinary and exotic fermions. However, it is possible to evade these bounds by considering the fine-tuned cases in which the mixing matrices F † α F α are diagonal. These correspond to those directions in parameter space in which each known ordinary fermion mixes with its own exotic fermion. If the strong assumption F ij L,R = 0 for i = j is made, one can write
F † α F α ij = s i α 2 δ i j α = L, R, (2.7) in which s i α 2 ≡ sin 2 θ i α , where θ i L(R)
is the mixing angle of the i th L-handed (Rhanded) light fermion. In this case the ordinary-exotic mixing is parametrized by one angle per each L-or R-handed charged fermion. The same angles enter also the expression for the CC measurables. We refer to Refs. [4,5] for the extension of the formalism to the CC sector. Even if the limits on FCNC processes are not effective to constrain these particular fine-tuned mixings, still all the s i α 2 can be constrained by looking at the high precision **data** involving CC and flavor-diagonal NC. The corresponding limits are given in Table 3.