GASP observations of the Crab pulsar taken during the 2017 campaign. The plot shows a 1-hour observation starting at 2017-09-29 02:20:01. The red line shows the linear polarization and the black line is the optical light curve referenced to the Jodrell Bank ephemeris, Lyne et al. (1993) and http://www.jb.man.ac.uk/~pulsar/crab.html. The linear polarization in the off-pulse region, from phase 0.7–0.9, is 37 ± 5% consistent with other measurements, see, e.g., Słowikowska et al. (2009)