  In understanding the composition and internal structure of asteroids, their
density is perhaps the most diagnostic quantity. We aim here to characterize
the surface composition, mutual orbit, size, mass, and density of the small
main-belt binary asteroid (939) Isberga. For that, we conduct a suite of
multi-technique observations, including optical lightcurves over many epochs,
near-infrared spectroscopy, and interferometry in the thermal infrared. We
develop a simple geometric model of binary systems to analyze the
interferometric data in combination with the results of the lightcurve
modeling. From spectroscopy, we classify Ibserga as a Sq-type asteroid,
consistent with the albedo of 0.14$^{+0.09}_{-0.06}$ (all uncertainties are
reported as 3-$\sigma$ range) we determine (average albedo of S-types is 0.197
$\pm$ 0.153, Pravec et al., 2012, Icarus 221, 365-387). Lightcurve analysis
reveals that the mutual orbit has a period of 26.6304 $\pm$ 0.0001 h, is close
to circular, and has pole coordinates within 7 deg. of (225, +86) in ECJ2000,
implying a low obliquity of 1.5 deg. The combined analysis of lightcurves and
interferometric data allows us to determine the dimension of the system and we
find volume-equivalent diameters of 12.4$^{+2.5}_{-1.2}$ km and
3.6$^{+0.7}_{-0.3}$ km for Isberga and its satellite, circling each other on a
33 km wide orbit. Their density is assumed equal and found to be
$2.91^{+1.72}_{-2.01}$ g.cm$^{-3}$, lower than that of the associated ordinary
chondrite meteorites, suggesting the presence of some macroporosity, but
typical of S-types of the same size range (Carry, 2012, P\&SS 73, 98-118). The
present study is the first direct measurement of the size of a small main-belt
binary. Although the interferometric observations of Isberga are at the edge of
MIDI capabilities, the method described here is applicable to others suites of
instruments (e.g, LBT, ALMA).
