A contribution to the study of asteroids with long rotational period

We report the results of new photometric and/or spectroscopic observations of 22 asteroids, selected among a sample of presumed slow rotators. All the available previous estimations of their rotation periods give values larger than 12 h. It has been recently suggested by Harris (1994) that some of these asteroids could be in a motion of non-principal axis rotation, due to a primordial excited state. The observations reported here were carried on at the Observatorio do Pico-dos-Dias (Brazil), at the Observatoire de Haute-Provence (France), and at the European Southern Observatory (Chile). More than 60 single night lightcurves were obtained. Rotation periods and the corresponding composite lightcurves were obtained for 5 objects. Spectroscopic observations were also carried on to increase the knowledge on these objects and verify the hypothesis of a common origin. We have obtained reflectivity spectra covering 0.5-0.9 mu m for 18 objects with orbital semi-major axes between 2.23 and 3.15 AU. For most of them (11) the spectra suggest a C-type composition, and for the others an S or E-type. Between the C-type asteroids, all but two show features associated with an aqueous alteration process. (C) 1999 Elsevier Science Ltd. All rights reserved.