  We present a study of low- and high-resolution ultraviolet, high-resolution
optical CAT/CES spectra and ultraviolet, optical and infrared photometry of the
peculiar supergiant HD101584. From the photometry we learn that the ultraviolet
and optical energy distribution cannot be fitted in a consistent way and we
need a model in which the UV and optical energy distribution are formed by
different gas. The Geneva photometry is best fitted to a B9II Kurucz model,
Teff=12000+-1000K and log g=3.0 +-1.0, with an extinction of E(B-V)=0.49+-0.05.
  The observed spectral features in the spectrum of HD101584 are classified in
eight different categories based on the velocity, shape of profile and the
identification. The high-excitation HeI(chi=20.87eV), NII(chi=18.40eV), CII
(chi=14.39eV) and NI (chi=10.29eV) optical absorption lines are formed in the
photosphere of a late B-star (e.g. B8-9I-II). These absorption lines show
radial velocity variations which are attributed to binary motion, with the
secondary being a white dwarf or a low-mass main sequence object. The
low-excitation P-Cygni lines in the optical and UV are formed in the wind. The
number density of absorption lines in the UV is so large that the wind spectrum
acts as an iron curtain in front of the B-star. The terminal velocity of the
wind of $v_{\infty}=100\pm30$ km~s$^{-1}~$ is consistent with the star being a
low-mass post-AGB star and the low effective gravity is attributed to the
presence of a nearby, unseen, secondary. We estimate a mass-loss rate of
\.{M}$\approx 10^{-8}$~M$_{\odot}$~yr$^{-1}$. Narrow absorption and emission
lines are observed which are formed in a circumsystem disk with a typical
radius of $10^{2} R_{\ast}$.
