  We investigate the mass-to-light ratios of stellar populations as predicted
by stellar population synthesis codes and compare those to
dynamical/gravitational measurements. In Bell & de Jong (2001) we showed that
population synthesis models predict a tight relation between the color and
mass-to-light ratio of a stellar population. The normalization of this relation
depends critically on the shape of the stellar IMF at the low-mass end. These
faint stars contribute significantly to the mass, but insignificantly to the
luminosity and color of a stellar system. In Bell & de Jong (2001) we used
rotation curves to normalize the relation, but rotation curves provide only an
upper limit to the stellar masses in a system. Here we compare stellar and
dynamical masses for a range of stellar systems in order to constrain the mass
normalization of stellar population models. We find that the normalization of
Bell & de Jong (2001) should be lowered by about 0.05-0.1 dex in M/L. This is
consistent with a Kroupa (2001), Chabrier (2003) or a Kennicutt (1983) IMF, but
does not leave much room for other unseen components.
