Third order corrections to the semileptonic b →c and the muon decays

We compute corrections of order α$^{3}$$_{s}$ to the decay b→cl¯ν taking into account massive charm quarks. In the on-shell scheme large three-loop corrections are found. However, in the kinetic scheme the three-loop corrections are below 1% and thus perturbation theory is well under control. We furthermore provide results for the order α$^{3}$$_{s}$corrections to b→ul¯ν and the third-order QED corrections to the muon decay which will be important input for reducing the uncertainty of the Fermi coupling constant G$_{F}$.