  We have obtained the first measures of the mass accretion rates on stars of
the Trapezium Cluster. They appear systematically lower than those of similar
stars in the Taurus-Auriga association. Together with premature disk
evaporation, dramatically revealed by the HST images of the Orion proplyds,
this result suggests that low mass stars in a rich cluster may be ``dwarfed''
by the influence of nearby OB stars. This feedback mechanism affects the IMF,
producing an excess of low mass stars and brown dwarfs. The observed frequency
of low mass objects in Orion vs. Taurus seems to confirm this scenario.
