Weak-singlet fermions: Models and constraints

We employ data from precision electroweak tests and collider searches to derive constraints on the possibility that weak-singlet fermions mix with the ordinary Standard Model fermions. Our findings are presented within the context of a theory with weak-singlet partners for all ordinary fermions and theories in which only third-generation fermions mix with weak singlets. In addition, we indicate how certain results can be applied more widely in theories containing exotic fermions