2006PASP..118..659L

We report R and I band observations for 204 late M and L dwarfs selected on the basis of Two Micron All Sky Survey JHK_sand photographic red colors, made with a telescope of modest aperture. It is shown that deep surveys covering these red bands can provide data sets that complement JHK_s, permitting a fairly good photometric classification system for L dwarfs. Due primarily to the disappearance of strong TiO opacities, R-I reaches a maximum at late M type and turns blueward for subtypes M9 to about L3. Apart from a small plateau at L0-L2, the I-K_scolor remains as a monotonic measure of spectral type or temperature over this range, and likewise for M dwarfs. For late L types, both colors probably get redder again, although the accuracy of our data and number of objects do not give us robust conclusions by L6-L8. It is also interesting to look at the dispersions of the IJK_sbands at a given spectral type. It is widely believed that this dispersion is caused by object-to-object variations in the amount, location, or other properties of dust or clouds. We find a moderately larger spread for the J-K_scolor than for I-J.