  We report on the discovery of strong intensity variations in the high
rotational lines of abundant molecular species towards the archetypical
circumstellar envelope of IRC+10216. The observations have been carried out
with the HIFI instrument on board \textit{Herschel}\thanks{\textit{Herschel} is
an ESA space observatory with science instruments provided by European-led
Principal Investigator consortia and with important participation from NASA}
and with the IRAM\thanks{This work was based on observations carried out with
the IRAM 30-meter telescope. IRAM is supported by INSU/CNRS (France), MPG
(Germany) and IGN (Spain)} 30-m telescope. They cover several observing periods
spreading over 3 years. The line intensity variations for molecules produced in
the external layers of the envelope most probably result from time variations
in the infrared pumping rates. We analyze the main implications this discovery
has on the interpretation of molecular line emission in the envelopes of
Mira-type stars. Radiative transfer calculations have to take into account both
the time variability of infrared pumping and the possible variation of the dust
and gas temperatures with stellar phase in order to reproduce the observation
of molecular lines at different epochs. The effect of gas temperature
variations with stellar phase could be particularly important for lines
produced in the innermost regions of the envelope. Each layer of the
circumstellar envelope sees the stellar light radiation with a different lag
time (phase). Our results show that this effect must be included in the models.
The sub-mm and FIR lines of AGB stars cannot anymore be considered as safe
intensity calibrators.
