### Propagation de la 2-birationalité

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-extension K’/K is linearly disjoint from the cyclotomic ℤ₂-extension ${K}^{c}/K$, we prove that K’/K is at most quadratic. Furthermore, we construct infinite towers of such 2-extensions.