... Telescope

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under contract No. NAS5-26555.

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... FGS

Figures 1 to 12 are only available in electronic form at http://www.edpsciences.org

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...2,

Invited researcher at the Osservatorio Astronomico di Torino.

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... orbit

The displacement due to the orbital motion of the targets themselves during each scan were negligible.

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... length

The contribution of the b and c semiaxes to the projected ellipse measured in the sky around the maximum of the light-curve is mainly a function of the aspect angle, as explained above. For instance, in the limiting case of an object observed at a perfectly polar view (aspect angle  $\xi = 0^{\circ}$), no indication of the length of the smallest axis c would be possible for any possible rotational phase.

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