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 within 1 • .5 of the Pleiades' center – reaching a completeness magnitude I = 19.5 – with the aim of finding free-floating brown dwarfs. Teide 1, the best bona f ide brown dwarf discovered so far in the cluster (Rebolo, Zapatero Osorio & Martín 1995), arose as a result of a combined photometric and astrometric study of ∼1/4 of our covered area. The extension of our two-colour survey provides eight new additional brown dwarf candidates whose photometry is rather similar to that of Teide 1. Several of them are even fainter. Follow up low-resolution spectroscopy (Martín, Rebolo & Zapatero Oso-rio 1996) shows that one of them is indeed a Pleiades brown dwarf. Most of the remaining candidates are background late-M dwarfs which are contaminating our survey, possibly due to a small (previously unknown) cloud towards the cluster which affects some of our CCD fields. We did not expect any foreground M8–M9 field dwarf in our surveyed volume and surprisingly we have found one, suggesting that its number could be larger than inferred from recent luminosity function studies in the solar neighbourhood.