  The microlensing event OGLE-2011-BLG-0417 is an exceptionally bright lens
binary that was predicted to present radial velocity variation at the level of
several km/s. Pioneer radial velocity follow-up observations with the UVES
spectrograph at the ESO - VLT of this system clearly ruled out the large radial
velocity variation, leaving a discrepancy between the observation and the
prediction. In this paper, we further characterise the microlensing system by
analysing its spectral energy distribution (SED) derived using the UVES
spectrum and new observations with the ARCoIRIS (CTIO) near-infrared
spectrograph and the Keck adaptive optics instrument NIRC2 in the J, H, and Ks
bands. We determine the mass and distance of the stars independently from the
microlensing modelling. We find that the SED is compatible with a giant star in
the Galactic bulge and a foreground star with a mass of 0.94+/-0.09Msun at a
distance of 1.07+/-0.24kpc. We find that this foreground star is likely the
lens. Its parameters are not compatible with the ones previously reported in
the literature (0.52+/-0.04Msun at 0.95+/-0.06kpc), based on the microlensing
light curve. A thoughtful re-analysis of the microlensing event is mandatory to
fully understand the reason of this new discrepancy. More importantly, this
paper demonstrates that spectroscopic follow-up observations of microlensing
events are possible and provide independent constraints on the parameters of
the lens and source stars, hence breaking some degeneracies in the analysis.
UV-to-NIR low-resolution spectrographs like X-SHOOTER (ESO - VLT) could
substantially contribute to this follow-up efforts, with magnitude limits above
all microlensing events detected so far.
