Abstract　 　　  

  This seminar presents recent results on the orbital characteristics and possible origin of close-in multi-planetary systems in the vicinity of the 2/1 mean-motion resonance. 

  First we review the architecture of Kepler systems and compare them with systems detected by radial velocity and direct imaging. We then discuss the main features of the near-resonant systems and outline some of the possible explanations for the observed asymmetry around the 2/1 resonance. 

  Finally, we show results of a series of hydrodynamical and N-body simulations and present a new scenario which may help explain the formation and past evolution of (at least) some of these systems.