  We investigate gravitational microlensing of point-like lenses surrounded by
diffuse gas clouds. Besides gravitational bending, one must also consider
refraction and absorption phenomena. According to the cloud density, the light
curves may suffer small to large deviations from Paczynski curves, up to
complete eclipses. Moreover, the presence of the cloud endows this type of
microlensing events with a high chromaticity and absorption lines recognizable
by spectral analysis. It is possible that these objects populate the halo of
our galaxy, giving a conspicuous contribution to the fraction of the baryonic
dark matter. The required features for the extension and the mass of the cloud
to provide appreciable signatures are also met by several astrophysical
objects.
