  Context: The dwarf planet (1) Ceres - next target of the NASA Dawn mission -
is the largest body in the asteroid main belt; although several observations of
this body have been performed so far, the presence of surface water ice is
still questioned.
  Aims: Our goal is to better understand the surface composition of Ceres, and
to constrain the presence of exposed water ice.
  Methods: We acquired new visible and near-infrared spectra at the Telescopio
Nazionale Galileo (TNG, La Palma, Spain), and reanalyzed literature spectra in
the 3-$\mu$m region.
  Results: We obtained the first rotationally-resolved spectroscopic
observations of Ceres at visible wavelengths. Visible spectra taken one month
apart at almost the same planetocentric coordinates show a significant slope
variation (up to 3 %/10$^3\AA$). A faint absorption centered at 0.67 $\mu$m,
possibly due to aqueous alteration, is detected in a subset of our spectra. The
various explanations in the literature for the 3.06-$\mu$m feature can be
interpreted as due to a variable amount of surface water ice at different
epochs.
  Conclusions: The remarkable short-term temporal variability of the visible
spectral slope, and the changing shape of the 3.06-$\mu$m band, can be hints of
different amounts of water ice exposed on the surface of Ceres. This would be
in agreement with the recent detection by the Herschel Space Observatory of
localized and transient sources of water vapour over this dwarf planet.
