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Polycyclic aromatic hydrocarbons (PAHs) produce characteristic infraredemission bands that have been observed in a wide range of astrophysicalenvironments, where carbonaceous material is subjected to ultraviolet (UV)radiation. Although PAHs are expected to form in carbon-rich AGB stars, theyhave up to now only been observed in binary systems where a hot companionprovides a hard radiation field. In this letter, we present low-resolutioninfrared spectra of four S-type AGB stars, selected from a sample of 90 S-typeAGB stars observed with the infrared spectrograph aboard the Spitzer satellite.The spectra of these four stars show the typical infrared features of PAHmolecules. We confirm the correlation between the temperature of the centralstar and the centroid wavelength of the 7.9 {\mu}m feature, present in a widevariety of stars spanning a temperature range from 3 000 to 12 000 K. Three offour sources presented in this paper extend this relation towards lowertemperatures. We argue that the mixture of hydrocarbons we see in these S-starshas a rich aliphatic component. The fourth star, BZ CMa, deviates from thiscorrelation. Based on the similarity with the evolved binary TU Tau, we predictthat BZ CMa has a hot companion as well.

When an old star smolders: On the detection of hydrocarbon emission from S-type AGB stars