Upper panel: Stokes parametres of SN 2008D/XT 080109 and SN 2007uy, once the GIP correction has been included. The cloud of measurements representing SN 2007uy seems to be more compact than the SN 2008D/XT 080109 one. The different lines represent the linear fits to the SN 2008D/XT 080109 Stokes parametres obtained when the CAHA (dot-dashed), VLT (continuous), NOT (dotted) and all the data (VLT+CAHA+NOT, long-dashed) are considered. The directions of all the straight lines are statistically consistent (see Table 4), which might indicate the presence of a preferential axis. This possible evidence is strengthened when SN 2007uy is used as reference (see below). Bottom panel: Stokes parametres of SN 2008D/XT 080109 and SN 2007uy, if SN 2007uy is kept fixed at its barycentre. The directions of all the straight lines are statistically more consistent than in the upper panel (compare the internal consistency of the different orientations in Tables 3 and 4), reinforcing the possible presence of an instrumental-independent dominant symmetry axis present in the SN 2008D/XT 080109 data.