Pergamon Planet. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH Space Sci., Vol. 45, No. 11, pp. 1423-143?, 1997 0 1997 Elsevier Science Ltd. All rights reserved. Printed in Great Britain 0032-0633/97 $17.00+0.00 zyxwvuts PII: SOO32-0633(97)00121-9 Rotational properties of main belt asteroids : photoelectric and CCD observations of 15 objects* M. Flor~zak,‘,*,~ E. Dotto, 3,4 M. A. Barucci,3 M. Birlan,’ A. Eriksoq6 M. Fulchignoni,3S7 A. Nathues, L. Perret and P. Thebault3 ‘ON/CNPq, Dep. Astrofisica, 20921 Rio de Janeiro, Brazil ‘CEFET, Dep. Fisica, 80000 Curitiba, Brazil 30bservatoire de Paris, 92195 Meudon Principal Cedex, France %niversitd di Padova, Dip. di Fisica, V. Marzolo 8, 35131 Padova, Italy ‘Astronomical Institute of the Romanian Academy, str. Cutitul de Argint 5, Bucharest 28, Romania 6DLR, Institute of Planetary Exploration, Rudower Chausse 5, D-12489 Berlin, Germany ‘UniversitB Paris 7, Paris, France Received 19 March 1997; accepted 20 May 1997 Introduction The knowledge of the asteroid spin rate is an important tool to gain information on the collisional evolution state *Based on observations carried out at the European Southern Observatory (ESO), La Silla, Chile. Correspondence to: M. A. Barucci. E-mail: BARUCCI@ obspm.fr. of the asteroid population. Binzel et al. (1989) showed that the intermediate size range is important in terms of collisional evolution: the limit between 1OOkm and 125 km is considered as very representative because it seems to be the transition region between the larger pri- mordial asteroids and the population of the smaller objects which are supposed to be the fragments, results of collisional events. Fulchignoni et al. (1995), analysing the rotational rate distribution for a sample of 516 main belt asteroids, found, for small objects (D I 50 km), the super- imposition of three sub-populations : the more populated (similar to the distribution of larger objects) and the slow and the rapid rotator ones. Harris (1996), analyzing the rotational spin of asteroids with mean diamet& D I 10 km, pointed out an absence of very rapid rotators, and an excess of slow rotators : the first characteristic (cut-off for period of less than 2.25 h) seems to imply a “rubble pile” structure, while the excess of slow rotators can be due to a state of non-principal axis rotation, or “tumbling” (Harris, 1994). In order to enlarge the available data set of asteroid spins, we are carrying out a long-term observing program. In this paper we present 64 lightcurves of 15 asteroids, obtained during 1996 by CCD observations at the 1.2 m telescope of the Haute Provence Observatory (France) and at 0.9 m Dutch, 1.5m Danish and 0.6 m Bochum telescopes of the European Southern Observatory (ESO, La Silla, Chile) and by photoelectric observations carried out with the 0.5m telescope at ESO. For all the objects, with the exception of 783 Nora, lightcurves were measured for the first time. The survey has been carried out on small asteroids : 13 out of the 15 observed objects have a diameter D < 50 km, while the remaining objects have a diameter of about 90 km. Two. of the observed asteroids (732 Tjilaki and 2446