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SCIENTIFIC ABSTRACT The discovery of cool material around main-sequence stars such as Vega and Beta Pictoris has maybe been most the impacting new result from the IRAS mission, and is widely seen as one of the most promising new facts that may lead to a better understanding of the formation of planetary systems. The spectroscopic capabilities of ISO provide the ideal tool for studying the circumstellar emission in Vega-type stars in more detail. Measurements from the ground have provided a first hint that cometary-like silicate emission is present around Beta Pic, HD 98800, and 51 Oph. With SWS, this feature can be studied much more accurately and can be detected in other stars as well. The precise determination of the onset of the IR excess provides an important test on the size of the circumstellar grains. Important emphasis is put on the study of the evolution of the proto-planetary disks from YSOs (studied in the Wesselius et al. proposal) to main-sequence stars. We have chosen to focus on objects in the 2-solar-mass range (Herbig Ae/Be stars), because it appears that for such objects the transition toward the main sequence can be studied in objects that are relatively free from background emission. The energy distributions of these objects suggest that the IR excess first weakens in the mid-IR region, i.e. the spectral region covered by SWS. An interesting possibility is that this happens through the formation of larger bodies. OBSERVATION SUMMARY. Measurements with SWS will be performed for all stars for which the expected S/N ratio is sufficient. The goal of these observations is to detect and describe solid-state features and to determine accurately the onset of the IR excess. Only medium-resolution (R = 200) AOT-01 observations (full scans) are requested. For all objects a PHOT-S (AOT-40) 6-12 micron scan will be observed, in order to describe accurately the photospheric flux, and so to determine accurately the onset of the IR excess. For some stars these PHOT-S spectra are included in the relevant PHOT proposal. PHOT-P (AOT-03) and PHOT-C (AOT-22) data are requested for those stars and wavelength ranges for which the expected S/N ratio is not sufficient for SWS and LWS. Measurements with LWS (AOT 01; 43-197 mu full-grating spectra; 4 samples per resolution element) are requested in a more exploratory mode, since the far-IR emission of these stars is essentially unknown so far; an interesting possibility is the detection of the water-ice bands at 44 and 62 micron. We propose to observe those stars not considered in the Barlow et al. proposal and for which a reasonable S/N ratio is expected. The spatial extent of the excess at 20 micron is expected to ..LAYOUT 2 be small for most program stars. CAM data (filter 3, 6"/pix) are nevertheless requested in an exploratory mode; such data are not very time-consuming and may prove essential for detecting background sources that would complicate the analysis. The spatial extent of the sources at 60 micron will be probed with multi-aperture PHOT observations. We will use PHOT AOT's 03 (P2/11), 04 (P3/13) and 22 (C100 and C200).