We present synthetic spectra in the range 640−930 nm for a sample of very cool dwarfs. The computations were performed using the recent “dusty” model atmospheres by Tsuji (this volume) and by Allard (1999), and a synthesis code (Pavlenko et al. 1995) working under LTE conditions. The absorptions of oxides (TiO and VO) and hydrides (CrH, FeH and CaH) are an important source of opacity for the coolest M-dwarfs and early L-dwarfs. We show that the densities of TiO and VO molecules and the shape and strength of their bands are sensitive to the formation of atmospheric dust. The absence of TiO and VO molecular bands in mid and late L-type dwarfs can be explained by a process of depletion of Ti and V atom into grains. The alkali elements, Li, Na and K present strong lines in the red and far-red spectrum of these objects, with Na and K contributing significantly to absorb the emerging radiation.

In order to reproduce the global shape of the optical spectra, an additional opacity is required in the computations. We have modelled it with a simple law of the form ao (v/vo)N, with N = 4, and have found that this provides a sufficiently good fit to the data. This additional opacity could be due to molecular/dust absorption or to dust scattering. The equivalent widths and intensities of the alkali lines are significantly affected by this opacity.