O(G$^{2}_{\mu}$m$^{4}_{t}$) electroweak radiative corrections to the Zb$\overline{b}$ vertex

The leading heavy-top two-loop corrections to the \Zbbvertex are determined from a direct evaluation of the corresponding Feynman diagrams in the large \Mtlimit. %The calculation is done in the naive %dimensional regularization scheme with anticommuting $\ga_5$. The leading one-loop top-mass effect is enhanced by $[1+\GF\Mt~2(9-\pi~2/3)/(8\pi~2\sqrt{2})]$. Our calculation confirms a recent result of Barbieri et al.