The Effects of a Vector-like Doublet as the Fourth Generation to R_b

We study the effects of a vector-like SU(2) doublet quarks as the fourth generation on $R_b$ and $R_c$. By considering the constraint from the experimental data of forward-backward asymmetry, $A_{FB}^b$ and $A_{FB}^c$,as well as FCNC among light quarks, we show that there is an allowed region of parameters for $R_b$ but not for $R_c$.