Overionization in X-ray spectra: a new paradigm for Mixed-Morphology SNRs .

Mixed-morphologySNRsarecharacterized by a shell-like radio emission, a cen trally peaked X-ray morphology, and by interaction with molecular clouds. Many models have been proposed to explain these peculiar remnants, but their physical origin is still un clear. The recent discovery of overionized (i. e. recombining) ejecta in 3 mixed-morphology SNRs has dramatically challenged all the previous models and opened up new, unexpected scenarios. I review the main properties of these remnants and their peculiar X-ray spectral properties. I also discuss the hydrodynamic model developed to explain the presence of overionized ejecta in W49B and present a list of open issues that still need to be clarified