Abstract
We describe how the large-scale geometry of the circumstellar envelopes of asymptotic giant branch stars can be used to probe the presence of unseen stellar companions. A nearby companion modifies the mass loss by gravitationally focusing the wind towards the orbital plane, and thereby determines the shape of the envelope at large distances from the star. Using available simulations, we develop a prescription for the observed shapes of envelopes in terms of the binary parameters, envelope orientation and type of observation. The prescription provides a tool for the analysis of envelope images at optical, infrared and millimetre wavelengths, which can be used to constrain the presence of companions in well-observed cases. We illustrate this approach by examining the possible role of binary companions in triggering the onset of axisymmetry in planetary nebula formation. If interaction with the primary leads to axisymmetry, the spherical haloes widely seen around newly formed nebulae set limits on the companion mass. Only low-mass objects may orbit close to the primary without observable shaping effects: they remain invisible until the interaction causes a sudden change in the mass-loss geometry.