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High-resolution interferometric observations of the circumstellarenvironments of AGB stars show a variety of morphologies. Guided by the unusualcarbon monoxide line profile of the AGB star EP Aquarii, we have observed itscircumstellar environment with ALMA band 6 in cycle 4. We describe themorphological complexity of the CO, SiO and SO2 molecular emission. The COemission exhibits the characteristics of a bi-conical wind with a bright nearlyface-on spiral feature around the systemic velocity. This is the firstconvincing detection of a spiral morphology in an O-rich wind. Based on theoffsets of the centers of the two bi-conical wind hemispheres, we deduce theposition angle of the inclination axis to be ~150 deg measured counterclockwisefrom north. Based on the velocity width of the spiral signature we estimate theinclination angle of the system to be between 4 deg and 18 deg. The centralemission zone exhibits a morphology that resembles simulations modelling thespiral-inducing wind-Roche-lobe-overflow mechanism. Though the spiral may be a(companion-induced) density enhancement in the stellar outflow, the extremelynarrow width of the spiral signature in velocity space suggests that it may bea hydrodynamical perturbation in a face-on differentially rotating disk. TheSiO emission does not show the spiral, but exhibits a local emission voidapproximately 0.5'' west of the continuum brightness peak. We hypothesise thatthis may be a local environment caused by the presence of a (stellar) companionwith a mass of at most 0.1 Msol, based on its non-detection in the continuum...

An unusual face-on spiral in the wind of the M-type AGB star EP Aquarii