The evolved (post-AGB) stars HR4049 and HD213985 are
  exceptionnal in the sense that they display circumstellar
  UV and optical extinction and IR emission.  They provide a
  rare opportunity to correlate dust features and so to
  identify dust carriers.  Moreover, both extinction and
  emission are periodically variable: the periods (430 and
  260 days) are orbital periods.
  HR4049 has a variable far-UV extinction rise and displays
  variable IR (PAH) emission bands.  Correlating both (IUE
  and ISO) will prove or disprove the claim that the far-UV
  extinction is due to PAHs.
  HD213985 has a constant far-UV extinction, but a variable
  220nm dip.  The IR emission, as measured so far, appears
  featureless but also variable.  Interestingly, we have
  observed circumstellar CH+ absorption for this star.
  Correlating UV, optical and IR observations of this star
  may put unprecedented constraints on the carrier(s) of the
  ubiquitous but still unexplained UV absorption dip in the
  interstellar medium.
  In both stars, the optical (linear) extinction component
  varies with the orbital period.  Simultaneous optical
  observations will be performed.
  Our aim with ISO is then to monitor the infrared emission
  of the circumstellar dust of both stars, with data well
  sampled over the known orbital periods.
  Both stars occur in Rens Waters' central program on
  post-AGB stars.  I am a co-I of this central program, and
  Rens Waters will be a co-I in this monitoring program.