tokens: it was about 3 per cent) to about 4.5 per cent at the time of our observations. In this spectral region, we have not detected any change between 2015 and 2018, but data obtained in 1995 were in between the values measured in the two epochs. At 6800 Å, data obtained by Putney ( 1995 ) show the lowest values among all data sets (∼1 per cent). Our BBP measurements in the red (4.0 per cent integrated over a R filter) are consistent with the relatively recent BBP measurement of Butters et al. ( 2009 ), who had detected a signal of 4.06 ± 0.16 per cent. In conclusion, we have clearly detected a change of 1.5 per cent over a 40+ yr interval, but found no evidence of change over a time-scale of the order of three years. 8.3 Linear polarization No large changes of the fraction of linear polarization are detected either on a short (3-yr) or long (40-yr) time-scale. However, over a 40–45 yr interval of time, the position angle has definitely changed by more than 10° both in the blue and the red. In the following, we look at the various linear polarization spectra and their differences in more detail. On the long time-scale sampled by the difference between the 1970s data and our recent ISIS spectra (40 yr), there definitely are changes. In particular, below 4000 Å, the recent level of linear polarization sampled by Q / I seems to have decreased, while that sampled by U / I is larger. This corresponds to a very significant rotation of the position angle of the linear polarization away from the value of ∼20° found in the 1970s to about 33° in recent times. We note that the measurement by West ( 1989 ) obtained 1986 suggests also a decrease of linear polarization compared to the 1970s (3.19 per cent in the B filter, for a position angle of 19.1 ± 0.8) but as discussed in Section 4.3 , these kinds of comparison should be made with caution. It is notable that although the circular polarization is quite different from zero throughout the visible spectrum, the non-zero linear polarization is present essentially below 5500 Å, with possible low-level features suggested by the Steward spectrum, but largely absent from the lower resolution MCSP data. The position angle in the red as measured in the 1970s was around 110°, with a fair amount of scatter due to the small but not quite vanishing amplitude of Q and U in that region. In the ISIS data, both Q and U are also close to zero, therefore the position angle has a large uncertainty, but is consistent with a 10°–30° rotation from the value measured in the 1970s. We conclude that linear polarization measurements are certainly variable over a time-scale of decades, mostly in terms of a rotation, while no significant variation has been detected on time-scale of three years. A rotation constant in wavelength provides a first rough description of the change in linear polarization. 9 CONSTRAINTS ON THE ROTATION PERIOD OF Grw+70° 8247 Measurements of the position angle of the broad-band linear polarization of Grw+70°
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