  We carried out high-sensitivity spectropolarimetric observations of the
central star of the Red Rectangle proto-planetary nebula with the aim of
constraining the mechanism that gives its biconical shape. The stellar light of
the central binary system is linearly polarised since it is scattered on the
dust particles of the nebula. Surprisingly, the linear polarisation in the
continuum is aligned with one of the spikes of the biconical outflow. Also, the
observed Balmer lines as well as the Ca II K lines are polarised. These
observational constraints are used to confirm or reject current theoretical
models for the shaping mechanism of the Red Rectangle. We propose that the
observed polarisation is very unlikely generated by a uniform biconical stellar
wind. Also, the hypothesis of a precessing jet does not completely match the
observations since it will require a jet aperture larger than that of the
nebula.
