The surprisingly carbon-rich environment of the S-type star W Aql
  W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental
abundance ratio C/O$\approx$0.98 and reported circumstellar molecular
abundances intermediate between those of M-type (C/O$<$1) and C-type (C/O$>$1)
AGB stars. This intermediate status is considered typical for S-type stars,
although our understanding of the chemical content of their circumstellar
envelopes (CSEs) is currently rather limited. We performed observations in the
frequency range 159-268 GHz with the APEX telescope and make abundance
estimates through comparison to available spectra towards some well-studied AGB
stars and based on rotational diagram analysis in the case of SiC2. We conclude
that W Aql's CSE appears considerably closer to that of a C-type AGB star than
to that of an M-type AGB star. In particular, we detect emission from C2H,
SiC2, SiN, and HC3N, molecules previously only detected towards the CSEs of
C-type stars. This conclusion, based on the chemistry of the gaseous component
of the CSE, is further supported by reports in the literature on the presence
of atmospheric molecular bands and spectral features of dust species typical
for C-type AGB stars. Although our observations mainly trace species in the
outer regions of the CSE, our conclusion matches closely that based on recent
chemical equilibrium models for the inner wind of S-type stars: the atmospheric
and circumstellar chemistry of S-type stars likely resembles that of C-type AGB
stars much more closely than that of M-type AGB stars. Further observational
investigation of the gaseous circumstellar chemistry of S-type stars is
required to characterise its dependence on the atmospheric C/O. Non-equilibrium
chemical models of the CSEs of AGB stars need to address the particular class
of S-type stars and the chemical variety that is induced by the range in
atmospheric C/O.
