  Aim : In order to study the history of mass loss in extreme OH/IR stars, we
observed a number of these objects using CO as a tracer of the density and
temperature structure of their circumstellar envelopes.
  Method : Combining CO observations from the Herschel Space Observatory with
those from the ground, we trace mass loss rates as a function of radius in five
extreme OH/IR stars. Using radiative transfer modelling, we modelled the dusty
envelope as well as the CO emission. The high-rotational transitions of CO
indicate that they originate in a dense superwind region close to the star
while the lower transitions tend to come from a more tenuous outer wind which
is a result of the mass loss since the early AGB phase.
  Result : The models of the circumstellar envelopes around these stars suggest
that they have entered a superwind phase in the past 200 - 500 years. The low
18O/17O (~ 0.1 compared to the solar abundance ratio of ~ 5) and 12C/13C (3-30
cf. the solar value of 89) ratios derived from our study support the idea that
these objects have undergone hot-bottom burning and hence that they are massive
M >= 5 solar-mass AGB stars.
