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Because they are extremely difficult to observe, cometary nuclei are poorly known objects in spite of their pristine nature and of their potential as pro- bes to the origin and evolution of the solar system. We propose to study the nuclei of up to 10 short-period comets by measuring their thermal emission in the 8-15 micron spectral band as a function of time over approximately 10 hours (expected mean value of their rotational periods). As demonstrated by ground- based observations under favorable conditions of a few comets devoid of coma (e.g., comet P/Arend-Rigaux), the thermal light curve allows to determine, under minimal assumptions, the sizes and shapes of the nuclei and to study their rotational properties. We shall also observe, as a "by-product", the thermal emission from the innermost dust coma and shall study possible dust structures and follow their temporal evolution. Our top priority goes to 5 cometary nuclei for which we have a similar and complementary program with the Hubble Space Telescope: P/Borrelly (approved HST program 5631), P/d'Arrest, P/Jackson-Neujmin, P/Honda-Mrkos-Pajdusakova and P/Wirtanen for which we submitted a proposal for the forthcoming cycle 5 and finally P/Kopff which we shall propose to observe during cycle 6. Although simultaneous observations are excluded because of very different constraints, combining the thermal (ISO) and the visible (HST) observations, after proper correction for different view- ing geometries, will allow to obtain additional information and constraints on these nuclei. In particular, it will be possible to derive their albedo which will give us information on their degree of mantling and on the composi- tion of the mantle. Extending our knowledge and understanding of cometary nuclei as made uniquely possible by ISO (and HST) is essential in the context of the space exploration of primitive bodies (ROSETTA and CONTOUR missions).