Debiased fractional polarization (top) and optical polarization plane direction (bottom) as a function of time. Time is given in both the observer’s and the source’s frame. Red points are the measurements likely not affected by the morning twilight (see Sect. 3). The blue points are those most likely affected by twilight. Errorbars correspond to a 1σ confidence level. The horizontal blue line and the width of the shaded region correspond to the weighted mean and the 1σ error of the mean of the first five measurements.