{\bf Abstract:} The circumstellar medium of stars on the Asymptotic
Giant Branch (AGB) represents an environment in which many important
astrophysical mechanisms take place.  The complexity is evidenced by
the interplay between different physical and chemical processes. 
Thus, the study of circumstellar envelopes around AGB stars is a
challange to the most advanced observational techniques as well as the
most developed model works.

One of the aspects that has not yet been sorted out is the time
dependence of the mass loss mechanism operating during this stage of
evolution.  Certainly, there is a growing evidence for temporal
variations of the mass loss with different time scales:

In the first part of this thesis ({\bf Papers~I \& II}), a novel
observational technique for studying the circumstellar enviroment of
AGB-stars has been used.  The first optical direct and polarimetric
images of stellar light scattered by circumstellar material are
presented.  The difficulty involved in the observation/reduction
process limits its effectiveness to AGB stars having a large apparent
circumstellar medium ($\geq$ 10\arcsec).  A number of carbon stars
which have experienced episodic mass loss, as evidenced by the
detached shells seen around them in CO maps, were selected as targets.
Two stars were clearly detected in circumstellar scattered light.  The
analysis of the data suggests that both gas and dust scattering are
present.  The latter is evidenced by the high degrees of polarisation
found in the scattered light, pointing to small-sized dust particles
($\approx$ 0.1\micron) as the scattering agent.  The images have
revealed new aspects of the structure of the circumstellar medium
around these stars, e.g. the existence of multiple shells, and a
separation of dust and gas particles.

The dependence of the mass loss on stellar parameters is also of great
importance for the understanding of AGB stars in a broader context,
e.g. their contribution to the chemical evolution of galaxies:

In the second part of this thesis ({\bf Papers~III \& IV}), the mass
loss rates, circumstellar SiO abundances and gas expansion velocities
of a sample of M-type AGB variables are derived.  An extensive
modelling of circumstellar CO and SiO thermal radio line emission has
been performed.  A Monte Carlo code, which solves the molecular
excitation and the energy balance equation of the gas, has been used
for the purpose.  It is found no significant difference between the
irregulars and the semiregulars in terms of their mass loss
characteristics.  There is no dependence of the mass loss rate on the
period of pulsation or stellar temperature.  The mass loss rate and
the gas expansion velocity though correlates well.  The sample
contains a number of sources with very low expansion velocities, and
also a number of sources with distinctly double-component profiles.  A
comparison with a similar sample of carbon stars is made. The
modelling of the SiO lines is aimed at a study of the importance of
gas depletion onto grains. The derived SiO abundances are, on average,
a factor of ten lower than the abundance expected from stellar
atmosphere chemistry. There is a trend of decreasing SiO abundance
with increasing mass loss rate (with an intriguing bimodal distribution
for the high mass loss rate Miras).  This trend is interpreted in
terms of depletion of SiO molecules by the formation of silicate
grains in the circumstellar envelopes, with an efficiency which
increases with the mass loss rate. It is observed that the SiO and CO
radio line profiles differ in shape, a result which has been
attributed to the influence of gas acceleration in the region where a
significant fraction of the SiO emission stems from.
