Our goal is to measure the masses of circumstellar disks associated
with young binary stars, and thereby to investigate the evolution of
disks in binary environments. The formation of binaries is accompanied
by the formation of protostellar disks. Dynamical theory predicts that a
binary embedded within a disk will evacuate a gap, producing circum-
stellar and circumbinary disks. The resonant truncation of these disks
will prevent the flow of material from the circumbinary disk to the
circumstellar disks. Continued accretion of circumstellar disk material
without replenishment would rapidly exhaust the circumstellar disks,
particularly in the closer binaries.
    Yet many pre-main-sequence binaries show evidence for circumstellar
disks, such as mid-infrared excesses and active accretion. Whether these
disks have surface densities typical of disks around single stars or are
depleted is not known. The survival of high surface densities in circum-
stellar disks would have implications for binary formation theories,
accretion disk dynamics, and planet formation.
    Mid-infrared observations are not sensitive to disk mass, except at
the very lowest disk surface densities. Observations at longer wave-
lengths where dust opacities are low are required. ISO observations of
continuum emission at 60 um and 120 um are uniquely capable of
measuring the masses of circumstellar disks in young binary stars.
Ground-based submillimeter telescopes cannot achieve the required
flux sensitivities.
    We propose ISO observations for a sample of pre-main-sequence binary
stars with a range in separation from 1 AU to 150 AU. We will determine
whether binaries with luminous mid-infrared excesses have high-surface-
density circumstellar disks. We will map circumstellar disk surface-
density as a function of binary separation, testing whether disks are
more readily depleted in close binaries. Finally, we will establish evo-
lutionary timescales to be compared with ISO findings for single stars.