Bounds on the mixing of the down-type quarks with vectorlike singlet quarks

We derive bounds on the mixing of the standard charge $\ensuremath{-}\frac{1}{3}$ quarks with vectorlike isosinglet quarks, as they exist in some extensions of the standard model. We make no assumptions about the unitarity or any other features of the mixing matrix. We find that the mixing is quite constrained: we are able to set bounds on all the extra parameters which arise in the mixing matrix (Cabibbo-Kobayashi-Maskawa matrix), except on two phases. The assumption that there exists only one exotic quark leads to some extra relationships among the parameters of the mixing matrix.