Towards Order alpha_s^4 Accuracy in tau-decays
  Recently computed terms of orders O(\alpha_s^4 n_f^2) in the perturbative series for the tau decay rate, and similar (new) strange quark mass corrections, are used to discuss the validity of various optimization schemes. The results are then employed to arrive at improved predictions for the complete terms order O(\alpha_s^4) and O(\alpha_s^5) in the massless limit as well as for terms due to the strange quark mass. Phenomenological implications are presented. 
P.A. Baikov, K.G. Chetyrkin and J.H. K\"uhn