Context :

Aims : To investigate the first high resolution optical spectrum of the B - type star , LS III +52 ^ { \circ } 24 , identified as the optical counterpart of the hot post - AGB candidate IRAS 22023+5249 ( I22023 ) .

Methods : We carried out detailed identifications of the observed absorption and emission features in the high resolution spectrum ( 4290 - 9015Å ) of I22023 obtained with the Utrecht Echelle Spectrograph on the 4.2m William Herschel Telescope .
Using Kurucz ’ s WIDTH9 program and the spectrum synthesis code , SYNSPEC , we determined the atmospheric parameters and abundances .
The photospheric abundances were derived under the LTE approximation .
The NEBULAR package under IRAF was used to estimate the electron temperature ( T _ { e } ) and the electron density ( N _ { e } ) from the [ N II ] and [ S II ] lines .

Results : We estimated T _ { eff } =24000 K , log g=3.0 , \xi _ { t } =7 kms ^ { -1 } .
The derived CNO abundances suggest an evolved star with C/O < 1 .
P - Cygni profiles of hydrogen and helium indicate ongoing 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 derived nebular parameters ( T _ { e } =7000 K , N _ { e } =1.2 \times 10 ^ { 4 } cm ^ { -3 } ) are comparable to those measured in young compact planetary nebulae ( PNe ) .
The nebular expansion velocity was estimated to be 17.5 kms ^ { -1 } .

Conclusions : The observed spectral features , large heliocentric radial velocity ( - 148.31 \pm 0.60 kms ^ { -1 } ) , atmospheric parameters and chemical composition indicate that I22023 , at a distance of 1.95 kpc , is an evolved post - AGB star belonging to the old disk population .
The nebular parameters suggest that the central star may be evolving into a compact , young PN , similar to Hen3 - 1357 .