  Astrometric observations of resolved binaries provide estimates of orbital
periods and will eventually lead to measurement of dynamical masses. Only a few
very low mass star and brown dwarf masses have been measured to date, and the
mass-luminosity relation still needs to be calibrated.
  We have monitored 14 very low mass multiple systems for several years to
confirm their multiplicity and, for those with a short period, derive accurate
orbital parameters and dynamical mass estimates.
  We have used high spatial resolution images obtained at the Paranal, Lick and
HST observatories to obtain astrometric and photometric measurements of the
multiple systems at several epochs. The targets have periods ranging from 5 to
200 years, and spectral types in the range M7.5 - T5.5.
  All of our 14 multiple systems are confirmed as common proper motion pairs.
One system (2MASSW J0920122+351742) is not resolved in our new images, probably
because the discovery images were taken near maximum elongation. Six systems
have periods short enough to allow dynamical mass measurements within the next
15 to 20years. We estimate that only 8% of the ultracool dwarfs in the solar
neighborhood are binaries with separations large enough to be resolved, and yet
periods short enough to derive astrometric orbital fits over a reasonable time
frame with current instrumentation. A survey that doubles the number of
ultracool dwarfs observed with high angular resolution is called for to
discover enough binaries for a first attempt to derive the mass-luminosity
relationship for very low-mass stars and brown dwarfs.
