  Asteroid (175706) 1996 FG3 is a binary asteroid and the baseline target for
the proposed MarcoPolo-R sample-return mission. We present thermal IR
photometry obtained with the ESO VLT+VISIR together with optical photometry
obtained with the ESO NTT+EFOSC2 . An absolute visual magnitude H_V = 17.833
\pm 0.024 and phase parameter G = -0.041 \pm 0.005 is derived. The Near-Earth
Asteroid Thermal Model (NEATM) has been fitted to the measured fluxes to derive
a geometric visual albedo p_v = 0.046 \pm 0.014, effective diameter at the
observed aspect D_eff = 1.68 \pm 0.25 km, and beaming parameter {\eta} = 1.15
for phase angle {\alpha} = 11.7{\deg}. The Advanced Thermophysical Model (ATPM)
has been fitted to the measured fluxes to derive a more accurate effective
diameter D_eff = 1.71 \pm 0.07 km and albedo p_v = 0.044 \pm 0.004. Based on
the ATPM results, assuming the same albedo for primary and secondary, we derive
a primary mean spherical diameter D_p = (1.69 +0.18/-0.12) km, secondary
diameter D_s = 0.51 \pm 0.03 km, and a secondary orbital semi-major axis a =
(2.8 +1.7/-0.7) km. A low surface thermal inertia {\Gamma} = 120 \pm 50 J m^-2
s^-1/2 K^-1 was also derived, suggesting a dusty surface and raising questions
as to the binary formation mechanism of this asteroid. These physical
properties are used to predict a Yarkovsky drift in semi-major axis of (-60
+31/-45) m yr-1.
