Polarization degree (shown by the colour map) and direction of the polarization vectors around Mira A (right panels) and Mira B (left panels). Upper and lower panels: image in the NR and the cnt820 filters, respectively. The red dots mark the positions of the two stars. The vectors are drawn when the polarization degree is higher than 1.5%. The offsets in declination and right ascension are given with respect to the position of Mira A.