Non factorizable O(ααs) 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 diagrams 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 taken into account by integrating over the whole phase space. All calculations, including Bremsstrahlung, are done in dimensional regularization. The expansion coefficients of the large mass expansion are given in closed from. Their expansion in y = m(Z)(2)/4m(W)(2) in is agreement with the coefficients 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 reserved.