  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.
