  Asteroid (3200) Phaethon is a Near-Earth Apollo asteroid with an unusual
orbit that brings it closer to the Sun than any other known asteroid. Its last
close approach to the Earth was in mid-December 2017 and the next one will be
on October 2026. Previous rotationally time-resolved spectroscopy of Phaethon
showed that its spectral slope is slightly bluish, in agreement with its B/F
taxonomic classification, but at some rotational phases, it changes to slightly
reddish. Motivated by this result we performed time-resolved imaging
polarimetry of Phaethon during its recent close approach to the Earth. Phaethon
has a spin period of 3.604 hours and we found a variation of the linear
polarisation with rotation. This seems to be a rare case in which such
variation is unambiguously found, also a consequence of its fairly large
amplitude. Combining this new information with the brightness and colour
variation, as well as previously reported results from Arecibo radar
observations, we conclude that there is no variation of the mineralogy across
the surface of Phaeton. However, the observed change in the linear polarisation
may be related to differences in the thickness of the surface regolith in
different areas or local topographic features.
