  Aims. The carbon-rich asymptotic giant branch (AGB) star TX Piscium (TX Psc)
has been observed multiple times during multiple epochs and at different
wavelengths and resolutions, showing a complex molecular CO line profile and a
ring-like structure in thermal dust emission. We investigate the molecular
counterpart in high resolution, aiming to resolve the ring-like structure and
identify its origin. Methods. Atacama Large Millimeter/submillimeter Array
(ALMA) observations have been carried out to map the circumstellar envelope
(CSE) of TX Psc in CO(2-1) emission and investigate the counterpart to the
ring-like dust structure. Results. We report the detection of a thin,
irregular, and elliptical detached molecular shell around TX Psc, which
coincides with the dust emission. This is the first discovery of a
non-spherically symmetric detached shell, raising questions about the shaping
of detached shells. Conclusions. We investigate possible shaping mechanisms for
elliptical detached shells and find that in the case of TX Psc, stellar
rotation of 2 km/s can lead to a non-uniform mass-loss rate and velocity
distribution from stellar pole to equator, recreating the elliptical CSE. We
discuss the possible scenarios for increased stellar momentum, enabling the
rotation rates needed to reproduce the ellipticity of our observations, and
come to the conclusion that momentum transfer of an orbiting object with the
mass of a brown dwarf would be sufficient.
