  We report on the results of a six-month photometric study of the main-belt
binary C-type asteroid 121 Hermione, performed during its 2007 opposition. We
took advantage of the rare observational opportunity afforded by one of the
annual equinoxes of Hermione occurring close to its opposition in June 2007.
The equinox provides an edge-on aspect for an Earth-based observer, which is
well suited to a thorough study of Hermione's physical characteristics. The
catalog of observations carried out with small telescopes is presented in this
work, together with new adaptive optics (AO) imaging obtained between 2005 and
2008 with the Yepun 8-m VLT telescope and the 10-m Keck telescope. The most
striking result is confirmation that Hermione is a bifurcated and elongated
body, as suggested by Marchis et al., (2005). A new effective diameter of 187
+/- 6 km was calculated from the combination of AO, photometric and thermal
observations. The new diameter is some 10% smaller than the hitherto accepted
radiometric diameter based on IRAS data. The reason for the discrepancy is that
IRAS viewed the system almost pole-on. New thermal observations with the
Spitzer Space Telescope agree with the diameter derived from AO and lightcurve
observations. On the basis of the new AO astrometric observations of the small
32-km diameter satellite we have refined the orbit solution and derived a new
value of the bulk density of Hermione of 1.4 +0.5/-0.2 g cm-3. We infer a
macroscopic porosity of ~33 +5/-20%.

Earth and Planetary Astrophysics