We present a detailed chemical analysis on the basis of high-resolution , high signal-to-noise optical spectra of two post-AGB objects IRAS 22223+4327 and IRAS 04296+3429 .
Both display the unidentified 21 \mu m feature in their IR-spectra .
The analysis is performed using HR 1865 ( F0Ib ) and HR 1017 ( F5Iab ) as reference stars .

The spectroscopic indicators provide accurate atmospheric parameters of T _ { eff } =6500 K , \log g =1.0 and \xi _ { t } =5.5 { km s ^ { -1 } } for IRAS 22223+4327 and T _ { eff } =7000 K , \log g =1.0 and \xi _ { t } =4.0 { km s ^ { -1 } } for IRAS 04296+3429 .
Our high-resolution data are inconsistent with the significantly lower temperatures deduced from spectral-type determinations in the literature based on low resolution spectra and highlight the need of high-resolution spectroscopy for the determination of accurate fundamental parameters of chemically peculiar supergiants .

Both photospheres are found to be metal-deficient with [ Fe/H ] = - 0.4 and - 0.7 respectively .
C and N are found to be overabundant .
Useful O-lines were only detected in the brighter IRAS 22223+4327 and the O abundance is found to follow the Fe deficiency : the C/O photospheric abundance is about 1.3 , but due to the lack of oxygen lines it is difficult to determine accurately .
This corroborates the fact that the carriers of the 21 \mu m feature are formed in a carbon rich circumstellar chemistry .
Moreover , these IRAS-stars have large overabundances of s-process-elements .
The mean abundance of all the measured s-process-elements is [ s/Fe ] =+1.0 for IRAS 22223+4327 and +1.4 for IRAS 04296+3429 .
The distribution of the s-process elements can best be described as due to a distribution of neutron exposures with a low mean neutron exposure of \tau _ { 0 } = 0.2 { mbarn ^ { -1 } } .
The 21 \mu m stars form an interesting sub-group in the total post-AGB sample of stars , not only for their IR characteristics , but also in a broader context of stellar ( chemical ) evolution theory .
They show , in contrast to other post-AGB stars , that the 3rd dredge-up has been efficient during their AGB evolution .
The mean neutron exposure is lower than expected for their metallicity .

The spectroscopic parameters found for the massive spectral analogues ( T _ { eff } =7500 K , \log g =2.0 and \xi _ { t } =3.0 { km s ^ { -1 } } for HR 1865 and T _ { eff } =6500 K , \log g =2.0 and \xi _ { t } =3.5 { km s ^ { -1 } } for HR 1017 ) agree well with the values found in the literature .
Both stars display an enrichment in Na , which is commonly observed in massive supergiants but theoretically not well understood .