  The experimental values of $R_b$ and $R_c$ are the only data which do not
seem to agree with Standard Model predictions. Although it is still premature
to draw any definite conclusions, it is timely to look for new physics which
could explain the excess in $R_b$ and deficit in $R_c$. We investigate this
problem in a simple extension of the Standard Model, where a charge $+2/3$
isosinglet quark is added to the standard spectrum. Upon the further
introduction of an extra scalar doublet, one finds a solution with interesting
consequences.
