1 Introduction

The main-belt asteroid (216) Kleopatra has been extensively observed during the last years by adaptive optics, speckle, radar, and interferometric techniques (e.g. Marchis et al. 1999; Hammergren et al. 2000; Merline et al. 2000; Ostro et al. 2000; Tanga et al. 2001). Such observations, with higher resolution than those previously obtained by Storrs et al. (1999) or Mitchell et al. (1995), show that this large M-type asteroid has a particular non-convex, dumbbell or bi-lobated shape. Radar delay-Doppler imaging technique has shown to be very powerful to determine the shape of near-Earth asteroids (e.g. Ostro et al. 1999; Benner et al. 1999; Hudson & Ostro 1999), and to put into evidence possible bifurcated or binary structures (e.g. Hudson & Ostro 1994; Benner et al. 2001; Margot et al. 2002). The Arecibo radar has been used to provide a 3-dimensional modelling of Kleopatra with the highest resolution presently possible from ground-based instruments. These data however do not directly provide spatially-resolved images and - since Kleopatra is in the main-belt - have limited signal-to-noise ratio yielding to an uncertainty in the model's shape of $\pm15~$km and, additionally, $\pm25$% in absolute size. The radar nominal-model has been shown to be consistent with ground-based adaptive optics observations (Hammergren et al. 2000; Merline et al. 2000), which confirm the bi-lobated shape of Kleopatra. Moreover, no significant albedo variations were detected on Kleopatra's surface (Hammergren et al. 2000).

Knowledge of the shapes of large asteroids in the main-belt is important because they are the result of a complex collisional history. Apart from "giant'' asteroids like Ceres and few others, most asteroids are the outcomes of catastrophic collisions, yielding to "rubble pile'' structures whose overall shapes may correspond to equilibrium figures (Farinella et al. 1981,1982). Collisions characterized by large angular momentum transfer may even produce binary systems that may eventually evolve into a single body having a bifurcated shape. An alternative could be the result of a gentle collision, in cases in which the shape of a "rubble pile'' could follow internal stress contours in a compressible material (Washabaugh & Scheeres 2001). It is therefore important to test the validity and limits of Kleopatra's shape models using data from independent observational techniques. This analysis can be done taking profit of a large set of available data, including photometric lightcurves, stellar occultation data, and high resolution interferometric observations recently obtained using the HST/FGS astrometer (Tanga et al. 2001).