Convexity properties of the quantum R\'enyi divergences, with   applications to the quantum Stein's lemma
  We show finite-size bounds on the deviation of the optimal type II error from its asymptotic value in the quantum hypothesis testing problem of Stein's lemma with composite null-hypothesis. The proof is based on some simple properties of a new notion of quantum R\'enyi divergence, recently introduced in [M\"uller-Lennert, Dupuis, Szehr, Fehr and Tomamichel, J. Math. Phys. 54, 122203, (2013)], and [Wilde, Winter, Yang, arXiv:1306.1586]. 
Mil\'an Mosonyi
quant-ph