Brown dwarfs and low-mass stars in the Pleiades and Praesepe:- Membership and binarity

We present near infrared J-, H- and K-band photometry and optical spectroscopy of low-mass star and brown dwarf (BD) candidates in the Pleiades and Praesepe open clusters. We find that Pleiades stars with K=10.5--13 are rather redder than the NextGen isochrones. We also identify this effect amongst alphaPer sources from the literature, but find no evidence of it for field stars from the literature. It is possible that the I-K colour of youthful stars is affected by the presence of magnetic activity. In the 2-colour diagram we find no colour change across the "M dwarf gap" (which would be indicative of dust formation). For Teff=2800--3150K, the colours of Praesepe and Pleiades sources differ, which may result from gravity sensitive molecular opacities. Praesepe sources cooler than 2500K agree with the dusty models, but Pleiades sources agree with the NextGen models down to 2000K. It is possible that dust formation does not begin until lower Teffs in sources with lower surface gravities. We also identify unresolved binaries in both clusters, and estimate mass ratios (q) for Pleiades BDs. Most of these have q>0.7, however, 3/18 appear to have lower q values. We determine the binary fraction (BF) for numerous mass ranges in each cluster, and find that it is generally rising towards lower masses. We find a BD BF of 50(+11/-10)%. We also find some evidence suggesting that the BF-q distribution is flat for 0.5-0.35Mo, in contrast to solar type stars.