A subsystem of a flat system of differential dimension 2 is flat

With an extended english abstract. A new version where the proof of the main theorem has been corrected using a new extended version of Rouchon's lemma.; A subsystem of a flat system of differential dimension at most 2 is flat. Furthermore, if such a flat system is stationary, we show that there exist flat outputs not depending on the time.; Un sous-système de dimension différentielle au plus 2 d'une extension plate est plate. Si un tel système plat est stationnaire, il ad-met des sorties plates indépendantes du temps.