2002AJ....123..466R

We present narrowband M photometry of nine low-mass dwarfs with spectral types ranging from M2.5 to L0.5. Combining the (L'-M') colors derived from our observations with data from the literature, we find colors consistent with a Rayleigh-Jeans flux distribution for spectral types earlier than M5, but enhanced F_3.8/F_4.7 flux ratios (negative [L'-M'] colors) at later spectral types. This probably reflects increased absorption at M' due to the CO fundamental band. We compare our results against recent model predictions and briefly discuss the implications.