  We analyze ISO/SWS spectra of the RV Tau star R Scuti. The infrared spectra
are dominated by H2O emission bands. The near- and mid-infrared excess is
attributed to H2O; the dust contribution is less important. We also identify
CO, SiO and CO2 bands. The various molecular emission bands originate from an
extended atmosphere, an atmosphere above the photosphere. The extended
atmosphere of R Sct is formed from matter which gradually have lifted up from
the photosphere through the pulsations of the star.
  In contrast to the abundant molecules around the star, the silicate dust
feature is weak and the dust mass-loss rate is only 10^{-11} solar mass per
year. This implies that there might be a process to inhibit dust formation from
molecules.
  RV Tau stars are commonly considered as post-AGB stars. While a detached dust
envelope around R Sct is consistent with such an interpretation, we show that
its period evolution is slower than expected. We argue that R Sct may be a
thermal-pulsing AGB star, observed in a helium-burning phase.
