  We report preliminary results of mid-infrared (MIR) and X-ray observations of
GRS 1915+105 that we carried out between 2004 October 2 and 2006 June 5. Our
main goals were to study its variability, to detect the presence of dust, and
to investigate the possible links between MIR and X-ray emissions.
  We performed photometric and spectroscopic observations of GRS 1915+105,
using the IRAC photometer and the IRS spectrometer mounted on the Spitzer Space
Telescope. We completed our set of MIR data with quasi-simultaneous high-energy
data obtained with RXTE and INTEGRAL.
  In the hard state, we detect PAH emission features in the MIR spectrum of GRS
1915+105, which prove the presence of dust in the system. The dust is confirmed
by the detection in the hard state of a warm MIR excess in the broadband
spectral energy distribution of GRS 1915 105. This excess cannot be explained
by the MIR synchrotron emission from the compact jets as GRS 1915+105 was not
detected at 15 GHz with the Ryle telescope. We also show that the MIR emission
of GRS 1915+105 is strongly variable; it is likely correlated to the soft X-ray
emission as it increases in the soft state. We suggest that, beside the dust
emission, part of the MIR excess in the soft state is non-thermal, and could be
due either to free-free emission from an X-ray driven wind or X-ray
reprocessing in the outer part of the accretion disc.
