The Contribution of Faint, Failed and Defunct Stars to the "Stellar"
  Masses of Galaxies
  A substantial fraction the stellar mass attributed to galaxies is invisible:
stars close to the hydrogen burning limit, brown dwarfs, white dwarfs, neutron
stars and black holes. These constituents do, however, gravitationally
micro-lens background quasars, thereby permitting measurement of the total
stellar contribution to the mass surface density along the line of sight. We
report the results of such a measurement using a sample of ten quadruply lensed
quasars. We discuss the prospects for improving upon this measurement with a
larger sample and describe efforts to find new quadruple lenses. If we invert
our argument and take the stellar mass to be known, we derive a value for the
fraction of the dark halo in MaCHOs (including 20 solar mass primordial black
holes) of somthing less than 10%, confirming the widely ignored result of
Mediavilla et al (2009).
