  We present the analysis of high angular resolution observations of the triple
Asteroid (87) Sylvia collected with three 8-10 m class telescopes (Keck, VLT,
Gemini North) and the Hubble Space Telescope. The moons' mutual orbits were
derived individually using a purely Keplerian model. We computed the position
of Romulus, the outer moon of the system, at the epoch of a recent stellar
occultation which was successfully observed at less than 15 km from our
predicted position, within the uncertainty of our model. The occultation data
revealed that the Moon, with a surface-area equivalent diameter
Ds=23.1$\pm$0.7km, is strongly elongated (axes ratio of
2.7$\pm$0.32.7$\pm$0.3), significantly more than single asteroids of similar
size in the main-belt. We concluded that its shape is probably affected by the
tides from the primary. A new shape model of the primary was calculated
combining adaptive-optics observations with this occultation and 40 archived
light-curves recorded since 1978. The difference between the
J2=0.024-0.009+0.016 derived from the 3-D shape model assuming an homogeneous
distribution of mass for the volume equivalent diameter Dv=273$\pm$10km primary
and the null J2 implied by the Keplerian orbits suggests a non-homogeneous mass
distribution in the asteroid's interior.
