Non factorizable O(alpha alpha(s)) corrections to the process Z -> b(b)over-bar

We evaluate non factorizable O(alpha alpha(s)) corrections to the process Z --> b (b) over bar due to the virtual t-quark. All two-loop vertex diagram s with W's and charged ghosts Phi's are included. They are evaluated in the large top-mass expansion up to the 10th order. Gluon Bremsstrahlung is tak en into account by integrating over the whole phase space. All calculations , including Bremsstrahlung, are done in dimensional regularization. The exp ansion coefficients of the large mass expansion are given in closed from. T heir expansion in y = m(Z)(2)/4m(W)(2) in is agreement with the coefficient s up to O(m(W)(6)/m(t)(6)) as given by Harlander et al. [Phys. Lett. B 426 (1998) 125]. (C) 1999 Published by Elsevier Science B.V. All rights reserve d.