In this paper we review the results of the denis survey on very low mass stars and brown dwarfs. The analysis of denis catalogs for 1500 square degrees has produced a sample of ∼ 100 very-late M dwarfs and 15 L dwarfs. Spectroscopy of these objects has established a spectroscopic classification sequence for L dwarfs, and determined the underlying effective temperature scale.

We use this sample to obtain the local luminosity function of the very low mass stars and brown dwarfs, with particular attention to correcting possible error sources and Malmquist-like biases. This first denis luminosity function has good statistical accuracy down to the limit between M and L dwarfs.

Introduction

Very low mass stars and brown dwarfs can be looked for around known brighter stars, in clusters, or in the general field, with advantages and disadvantages which have been repeatedly discussed in detail (for instance, Hambly 1998). Companion searches have historically identified the coolest object known at any given time, though usually not the least massive (which are found in clusters, where they haven't yet cooled much). Companion searches in the immediate solar neighbourhood also provide the information needed to correct cluster and field samples for the contribution of unresolved companions to more distant objects, and as such they are an essential complement to both field and cluster surveys. Cluster searches benefit both from an increased source density and from the much larger luminosity of younger brown dwarfs, and as a consequence they are sensitive to much lower mass objects (e.g. Zapatero-Osorio et al. 1999).