The history of the mass loss experienced by stars on the Asymptotic Giant 
Branch (AGB) is a key issue for describing the late stages of evolution of low 
and intermediate mass (1-6 Msun) stars, as well as an important ingredient for the 
characterization of the cosmic cycle of matter. Determining the mass loss rates 
of red giants is generally based on modelling radio molecular line profiles or 
infrared continuum energy distributions. However these methods are limited to 
the central parts of circumstellar shells, due to photodissociation (0.01 - 0.1 pc).
Besides, as the mass loss rates of AGB stars are variable, it has been 
difficult to establish a balance of the mass loss over the long periods of time 
that need to be considered (10^4-10^6 years). Two main sources of atomic hydrogen 
in a Circumstellar Environment (CSE) are expected: one directly from the hot 
atmosphere and the other from the photodissociation of molecular hydrogen in 
the envelope. Therefore the HI line at 21 cm should be a useful tracer of CSEs. 
But one has to face the confusion due to galactic emission and the faintness of 
the line. However, since 2001, using the upgraded Nancay Radiotelescope (NRT), 
Thibaut Lebertre and Eric Gerard readdressed this issue and detected several 
objects in emission. A detailed study of Y CVn could be performed thanks to 
favourable conditions. The profile shows that the line is composed of two 
features: a narrow and quite intense one (~400 mJy) and a faint (~15 mJy), 
broad, rectangular one. With the high spectral resolution profile, we were able 
to constrain the physical conditions in the CSE. I will present a model that 
describes the slowing down of the circumstellar outflow by external matter.