Propagation de la 2-birationalit\'e
  Let L/K be a 2-birational CM-extension of a totally real 2-rational number field. We characterize in terms of tame ramification totally real 2-extensions K'/K such that the compositum L'= LK' is still 2-birational. In case the 2-extensions K'/K is linearly disjoint from the cyclotomic Z2-extension Kc/K, we prove that K'/K is at most quadratic. In the other hand we construct infinite towers of such 2-extensions. 
Claire Bourbon (IMB), Jean-Fran\c{c}ois Jaulent (IMB)
math.NT