We report the discovery of two new carbon-rich proto-planetary nebulae (PPNs), IRAS Z02229+6208 and 07430+1115. Optical spectroscopy of these sources and another previously discovered PPN, IRAS 05431+0852, reveals the presence of C 2 and C 3 in absorption. All three objects have the spectra of G-K supergiants, consistent with the expectations of their being PPNs. New ground-based optical and infrared photometry, combined with the IRAS measurements, show double-peak spectral energy distributions for each; this suggests that the asymptotic giant branch (AGB) mass loss has ended and these objects are in the post-AGB phase of evolution. The remnant of the molecular envelope is detected in CO emission for the first time in all three objects, using the CO (3-2) line. The 3.3 and 11.3 μm emission features commonly attributed to the polycyclic aromatic hydrocarbon molecules have been detected in IRAS 07430+1115. Strikingly absent in IRAS 07430+1115, however, is the 21 μm emission feature, found in the other two and in all but one of the other PPNs known to show C 2 in absorption.
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Hrivnak et al. (1999) studied this question.