  It is commonly believed that winds of cool giants in their late evolutionary
stages are driven by radiative pressure on dust grains, but the actual grain
species responsible for driving winds of M-type AGB stars are still a matter of
debate (see H\"ofner, this volume). The optical properties of the dust
particles responsible for initiating the outflow affect the stellar spectra in
two ways: (i) indirectly, through their influence on the dynamical structure of
the atmosphere/envelope and the resulting molecular features, and (ii)
directly, by changes of the spectral energy distribution due to absorption and
scattering on dust grains. The qualitative differences in the energy
distributions of C-type and M-type AGB stars in the visual and near-infrared
regions suggest that the dust particles in oxygen rich atmospheres are
relatively transparent to radiation. By using detailed dynamical models of gas
and radiation combined with a simple description for the dust opacity (which
can be adjusted to mimic different wavelength dependencies and condensation
temperatures) and also by adjusting the fraction of the opacity that is treated
as true absorption, we investigate which dust properties produce synthetic
photometry consistent with observations. The goal of this study is to narrow
down the possible dust species that may be driving the winds in M-type AGB
stars.
