Program: GN-2006A-Q-75

We propose to use the high angular resolution provided by NIRI/ALTAIR to follow-up the orbits of binary main-belt systems already discovered (121 Hermione, 130 Elektra, 379 Huenna, 762 Pulcova, 1509 Esclangona) to improve their orbital elements. Determination of the precise orbital elements of the secondary can be done by multiple observations spanning large periods of time (on several years). Our group developed two independent visual methods to separate the geometrical orbit determination from the dynamical one including secular variations of the orbital plane and the pericenter. Without any assumptions, they led to the determination of the main orbital elements (a, P, e, i) of binary systems (see Marchis et al., 2004, 2005bcd). Additional observations will allow to detect unambiguously the precession of their orbit and constrain the distribution of mass in their interior (density, mass, porosity) giving direct insights on their formation processes. The recent discovery of two moonlets orbiting 87 Sylvia (Marchis et al.,2005e) indicates that multiple systems exist and should be discovered with large AO campaign.