Photometric surveys in nearby, young open clusters have provided a large amount of very low-mass stars and brown dwarfs over the last two decades. These clusters offer a number of advantages like known distance, metallicity and age, which make feasible the identification of such objects. Furthermore, deep searches do constitute one of the most direct means for measuring the mass function through the whole stellar (and brown dwarf) mass range. In this paper it will be reviewed the progress of recent work on several young open clusters leading to the findings of unambiguous brown dwarfs and very low mass stars approaching the substellar mass limit. These discoveries, particularly in the Pleiades, imply a rising mass function (α = 0.75 ± 0.25, dN/dM ∼ M−α) in the very low mass stellar and substellar domains down to 0.04 M⊙. The detection of reliable free-floating candidate members with estimated masses of only 0.04−0.015 M⊙ does provide substantial evidence on the formation of such low mass objects and thus, on the extension of the initial mass function down to the deuterium burning mass limit.

Introduction

Our knowledge of the low mass stellar content in open clusters has increased considerably during the last decade. For a relatively large amount of nearby young clusters, like α Per, Pleiades, Praesepe and Hyades, membership lists extending down to the hydrogen-burning limit (∼0.08 M⊙) are now available (see the reviews by Stauffer 1996, and Hambly 1998).