  Visible-light observations of Coronal Mass Ejections (CMEs) performed with
coronagraphs and heliospheric imagers (in primis on board the SOHO and STEREO
missions) have offered so far the best way to study the kinematics and
geometrical structure of these fundamental events. Nevertheless, it has been
widely demonstrated that only combination of multi-wavelength data (including
X-ray spectra, EUV images, EUV-UV spectra, and radio dynamic spectra) can
provide complete information on the plasma temperature and density
distributions, non-thermal motions, magnetic fields, and other physical
parameters, for both CMEs and CME-related phenomena. In this work, we analyze
three CMEs by combining simultaneous data acquired in the polarized visible
light by the LASCO-C2 coronagraph and in the UV H I Lyman-$\alpha$ line (1216
\AA) by the UVCS spectrometer, in order to estimate the CME plasma electron
density (using the polarization-ratio technique to infer the 3D structure of
the CME) and temperature (from the comparison between the expected and measured
Lyman-$\alpha$ intensities) along the UVCS field of view. This analysis is
primarily aimed at testing the diagnostic methods that will be applied to
coronagraphic observations of CMEs delivered by the Metis instrument on board
the next ESA-Solar Orbiter mission. We find that CME cores are usually
associated with cooler plasma ($T \sim 10^6$ K), and that a significant
increase of the electron temperatures is observed from the core to the front of
the CME (where $T > 10^{6.3}$ K), which seems to be correlated, in all cases,
with the morphological structure of the CME as derived from visible-light
images.
