Non factorizable O(ααs) corrections to the process Z→bb

We evaluate non factorizable O(ααs) corrections to the process Z→bb̄ due to the virtual t-quark. All two-loop vertex diagrams with Wʹs and charged ghosts Φʹs 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 form. Their expansion in y=mZ2/4mW2 is in agreement with the coefficients up to O(mW6/mt6) as given by Harlander et al. [Phys. Lett. B 426 (1998) 125].