  The HI line at 21 cm is a tracer of circumstellar matter around AGB stars,
and especially of the matter located at large distances (0.1-1 pc) from the
central stars. It can give unique information on the kinematics and on the
physical conditions in the outer parts of circumstellar shells and in the
regions where stellar matter is injected into the interstellar medium. However
this tracer has not been much used up to now, due to the difficulty of
separating the genuine circumstellar emission from the interstellar one.
  With the Nancay Radiotelescope we are carrying out a survey of the HI
emission in a large sample of evolved stars. We report on recent progresses of
this long term programme, with emphasis on S-type stars.
