BROWN DWARFS IN THE PLEIADES CLUSTER - A CCD-BASED R,I-SURVEY

We have obtained deep CCD R and I mosaic imaging of 578 arcmin(2) with in 1 degrees .5 of the Pleiades' center - reaching a completeness magn itude I = 19.5 - with the aim of finding free-floating brown dwarfs Te ide 1, the best bona fide brown dwarf discovered so far in the cluster (Rebolo, Zapatero Osorio & Martin 1995), arose as a result of a combi ned photometric and astrometric study of similar to 1/4 of our covered area. The extension of our two-colour survey provides eight new addit ional brown dwarf candidates whose photometry is rather similar to tha t of Teide 1. Several of them are even fainter. Follow up low-resoluti on spectroscopy (Martin, Rebolo & Zapatero Osorio 1996) shows that one of them is indeed a Pleiades brown dwarf. Most of the remaining candi dates are background late-M dwarfs which are contaminating our survey, possibly due to a small (previously unknown) cloud towards the cluste r which affects some of our CCD fields. We did not expect any foregrou nd M8-M9 field dwarf in our surveyed volume and surprisingly we have f ound one, suggesting that its number could be larger than inferred fro m recent luminosity function studies in the solar neighbourhood.