  We present optical polarimetry in the period May-August 2017 of the enigmatic
"dipping" star KIC 8462852. During that period three ~1% photometric dips were
reported by other observers. We measured the average absolute polarization of
the source, and find no excess or unusual polarization compared to a nearby
comparison star. We place tight upper limits on any change in the degree of
polarization of the source between epochs in- and out-of-dip of <0.1%
(8500-Ang.) and <0.2% (7050-Ang. and 5300-Ang.). How our limits are interpreted
depends on the specific model being considered. If the whole stellar disk were
covered by material with an optical depth of ~0.01 then the fractional
polarisation introduced by this material must be less than 10-20%. While our
non-detection does not constrain the comet scenario, it predicts that even
modest amounts of dust that have properties similar to Solar System comets may
be detectable. We note that the sensitivity of our method scales with the depth
of the dip. Should a future ~20% photometric dip be observed (as was previously
detected by Kepler) our method would constrain any induced polarization
associated with any occulting material to 0.5-1.0%.
