  Binary interactions have been proposed to explain a variety of circumstellar
structures seen around evolved stars, including asymptotic giant branch (AGB)
stars and planetary nebulae. Studies resolving the circumstellar envelopes of
AGB stars have revealed spirals, discs and bipolar outflows, with shaping
attributed to interactions with a companion. For the first time, we have used a
combined chemical and dynamical analysis to reveal a highly eccentric and
long-period orbit for W Aquilae, a binary system containing an AGB star and a
main sequence companion. Our results are based on anisotropic SiN emission, the
first detections of NS and SiC towards an S-type star, and density structures
observed in the CO emission. These features are all interpreted as having
formed during periastron interactions. Our astrochemistry-based method can
yield stringent constraints on the orbital parameters of long-period binaries
containing AGB stars, and will be applicable to other systems.
