We propose to measure the spectral energy distribution (SED) of an
important subgroup of relatively "old" pre-main-sequence stars (the UX
Ori-type stars, also called Algol-type young stars) which appear to be
in the transition phase between accretion and full disruption and
dissipation of their disk, an epoch at which planets may form. These
stars differ from other (younger?) pre-main-sequence stars because they
show evidence of variable circumstellar extinction, due to clouds of
dust in orbit around the star, and of high-velocity infalling gas,
probably produced by the dissipation of cometary bodies in star-grazing
orbits. A small number of these stars is currently monitored by ISO for
variability, also very likely associated with dust clouds arriving in
the vicinity of the star along elongated orbits.
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A crucial information  needed to understand the properties of
circumstellar matter (and one required by the analysis of variability
data) is good SEDs over the largest possible range of wavelengths. At
the time of the first ISO call, measurements of the SED of most of the
known UX Ori stars were planned in the various core programs on
pre-main-sequence stars. However, it appears now that the observations
obtained so far are limited to few stars and lack the systematic quality
we require. In particular, the variability data demonstrate the need to
obtain the overall SED at the same epoch. Moreover, the number of stars
identified as UX Ori-type stars has increased, and a statistical
approach to their properties has become possible.