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Published online by Cambridge University Press: 11 October 2004
We present results obtained with different adaptive optics systems(ADONIS, PUEO, NAOS), in either imaging or coronagraphic modes, in theframe of several surveys aimed at searching for very low mass starsand substellar objects around nearby M dwarfs as well as in youngcloseby associations. For the M dwarfs of the solar neighborhood, wehave obtained a very accurate mass-luminosity relation that we cancompare to the prediction of recent stellar structure models. We alsopresent multiplicity statistics of M dwarfs (binary fraction, periodand mass ratio distribution), which are crucial constraints forstellar formation scenarii. We also discuss results obtained on twoyoung closeby associations, MBM12 and Tucana-Horologium.