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tokens: from one epoch to the other, the polarised intensity distribution does not change significantly. The departure from axisymmetry is stronger in January 2015 but the location of the maxima and minima of the polarised flux does not change much between the two epochs. We divided the image in octants to facilitate our analysis and to minimise errors introduced by asymmetries of the dust envelope. Octant 1 is limited by the north and north-east directions and the numeration follows clock-wise. The value of the polarised intensity changes considerably for the different octants. In Table 2 we list the fraction of the polarised light measured per octant per filter. The ratio between the intensity from the octants with maximum and minimum polarised intensity is roughly 2.3 for and cntH in the first epoch and 3.7 for cntH in the second epoch. Table 2 Fraction of the polarised flux arising from the different octants for the image in and in cntH. 3. Analysis and modelling of the polarised light We now focus on our modelling efforts to derive the density radial profile of the grains that produce the scattered light. 3.1. Modelling approach We calculated spherically symmetric models using the continuum radiative-transfer code MCMax ( Min et al. 2009 ). The code calculates the direction-dependent scattering of radiation by dust grains and outputs images of the Stokes parameters. We convolved the Q and U images produced by the models with the PSF images. This typically caused a decrease by a factor of 1.5 of the output integrated polarised flux. This is because in the Q and U images the negative and positive lobes from a symmetrical envelope can overlap when the resolution is lowered. This leads to polarised signal in the images to cancel out. The poorer the angular resolution, the larger is the effect. The convolved Q and U images were then combined to obtain the polarised intensity for each model. We calculated the radial profile of the polarised intensity from the observations for each octant and compared these profiles independently to the models. We considered two different envelope structures: with dust grains distributed in a thin halo with constant density (model with no outflow), and with a dust density gradient given by , being the inner radius of the dust envelope and . We varied between 1.2 and 2.0 for both envelope structures and the halo thicknesses between 0.25 and 1.5 for the thin-halo scenario, where mas ( Norris et al. 2012 ). We use amorphous MgSiO grains with the optical constants obtained by Jäger et al. (2003) , as this dust species is one of the main candidates for driving the winds of oxygen-rich AGB stars (see, e.g., Bladh Höfner 2012 ). We have also experimented with optical constants for MgSiO and AlO, as discussed in Sect. 4.1.2 . To calculate the absorption opacities and the direction-dependent scattering properties, we approximate the actual size distribution of particle sizes by a distribution of hollow spheres ( Min et al.