  The first high-resolution (R~50,000) optical spectrum of the B-type star, LS
III +52 24, identified as the optical counterpart of the hot post-AGB candidate
IRAS 22023+5249 (I22023) is presented. We report the detailed identifications
of the observed absorption and emission features in the full wavelength range
(4290-9015 A) as well as the atmospheric parameters and photospheric abundances
(under the Local Thermodinamic Equilibrium approximation) for the first time.
The nebular parameters (Te, Ne) are also derived. We estimate Teff=24,000 K,
log g=3.0, xi=7 kms-1 and the derived abundances indicate a slightly
metal-deficient evolved star with C/O<1. The observed P-Cygni profiles of
hydrogen and helium clearly indicate on-going post-AGB mass loss. The presence
of [N II] and [S II] lines and the non-detection of [O III] indicate that
photoionisation has just started. The observed spectral features, large
heliocentric radial velocity, atmospheric parameters, and chemical composition
indicate that I22023 is an evolved post-AGB star belonging to the old disk
population. The derived nebular parameters (Te=7000 K, Ne=1.2x104 cm-3) also
suggest that I22023 may be evolving into a compact, young low-excitation
Planetary Nebula. Our optical spectroscopic analysis together with the recent
Spitzer detection of double-dust chemistry (the simultaneous presence of
carbonaceous molecules and amorphous silicates) in I22023 and other B-type
post-AGB candidates may point to a binary system with a dusty disk as the
stellar origin common to the hot post-AGB stars with O-rich central stars.
