Conclusions

We conlude that IRAS04296+3429 is a PPN candidate with a chemical composition which coincides with theoretical predictions for the post-AGB objects: very large excess of carbon and nitrogen are revealed. Moreover, the real excess (relative to iron) of heavy metals Y, Zr, Ba, La, Ce, Pr, Nd synthesized by the neutronization process indicates an effective third dredge-up and further confirms IRAS04296 to be in the advanced post-AGB evolution stage.

The emission of ${\rm C_2}$ molecular lines discovered in the spectra of IRAS04296 and its similarity to the emission of the Hale-Bopp comet allow us to suggest that in both cases the same mechanism (the resonance fluorescence) is responsible for the observed features.

Several strong absorptional features whose positions coincide with known diffuse interstellar bands (DIBs) are found in the spectrum of IRAS04296.

In addition, from the SED modelling of the spectral energy distribution we showed that 25${\mu}$m flux cannont be explained without assumption that the 30${\mu}$m emission feature is present in this source. Our recent ISO observation has detected the 30${\mu}$m band in this source.

Acknowledgements We are much indebted to the referee Hans van Winckel for critical reading the manuscript and valuable advice.

This work has been supported by project 1.4.1.1 of the Russian Federal Program ``Astronomy'' and grant 2.P03D.026.09 and 2.P03D.002.13 of the Polish State Committee for Scientific Research. One of us (R.Sz.) gratefully acknowledges the support from the Canadian Institute of Theoretical Astrophysics.