PHOTOSPHERIC COMPOSITION OF THE CARBON-RICH 21 MU M POST-AGB STARS IRAS 22223+4327 AND IRAS 04296+3429

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 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 Te =6500 K, log g=1.0 and t=5.5 kms 1 for IRAS 22223+4327 and Te =7000 K, log g=1.0 and t=4.0 kms 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 pa- rameters 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 m feature are formed in a carbon rich circumstellar chem- istry. Moreover, these IRAS-stars have large overabundances of s-process-elements. The mean abundance of all the mea- sured 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 distribu- tion of neutron exposures with a low mean neutron exposure of 0 =0 :2 mbarn 1 . The 21 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 (chem- ical) evolution theory. They show, in contrast to other post-AGB stars, that the 3rd dredge-up has been efficient during their AGB