tokens: the spectrum, particularly below 4400 Å; in the data of WR102, a broad peak in the degree of polarization (∼1 per cent deviation) and polarization angle (∼8° deviation) is seen between 7100 and 7600 Å. The latter feature cannot be a line effect as it is located in a region of the spectrum that is devoid of strong lines, and although its origin remains unknown (the details of our investigations are given in Section 3.2 ), we are confident it is of spurious nature. Given these considerations, the analysis presented in the rest of this study was performed without the spectral region below 4400 Å in WR93b, and the range 7100–7600 Å in WR102. Figure 1. Spectropolarimetric data of WR93b (left) and WR102 (right). The panels, from top to bottom, respectively, the degree of polarization (red) and best Serkowski law fit (black); the residuals after Serkowski fit removal (orange); the polarization angle (blue); Stokes I (grey) and line identification. The grey areas represent the discrepant regions of the spectrum described in Section 3.1 . Excluding these wavelength ranges, the polarization in the direction of WR93b and WR102 rises up to ∼4.5 per cent and ∼10 per cent, respectively. Their polarization angles have distinct behaviours: WR93b exhibits a constant polarization angle of θ = 28.2°, whereas the polarization angle of WR102 shows a downward trend that can be fit with a line of the form θ = −1.08 × 10 −3 λ + 18.4 deg. A non-zero Δθ/Δλ can be explained either by the superposition of intrinsic and interstellar polarization (Coyne 1974 ) or by the presence in the line of sight of dust clouds with different particle sizes and alignments (Coyne Gehrels 1966 ). The merit of these interpretations is discussed in Section 3.5 . 3.2 7300 Å feature in WR102 In Section 3.1 , we highlighted a prominent feature in WR102 spanning the wavelength range 7100–7600 Å. This peak was labelled as spurious and disregarded in our analysis. Here, we detail the steps taken in investigating the nature of this feature. First, we explored whether the spike in polarization is intrinsic to the data. The fact that the feature is not associated with any strong line (as would be expected for a line effect present in the WR star) is, however, inconsistent with this idea. Additionally, plotting the polarization, calculated for each of the 12 sets of observations at four half-wave retarder plate angles (see Fig. 2 ), reveals that the shape and orientation (peak or trough) are highly variable from set to set, and no one set is consistent with another. To formally check whether the data are spurious, we look at the difference in instrumental polarization corrections (Δε, see Maund 2008 ) in this region of the spectrum. This property is expected to be 0 for polarization signals ≤20 per cent, and deviations indicate that the polarization does not reflect a real signal (Maund 2008 ). In Fig. 2 , it is clearly visible that Δε is consistent with 0 across the spectrum but deviates
ner_tags: O O O O B-Wavelength I-Wavelength O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O B-Wavelength O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O B-CelestialObject O O O B-Wavelength I-Wavelength O B-CelestialObject O O O O O B-CelestialObject O O B-CelestialObject O O O O O O O O O O O O O O O B-Model I-Model I-Model O O O O B-Model I-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O B-CelestialObject O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O B-Formula I-Formula I-Formula O O O O O B-CelestialObject O O O O O O O O O O O O O O B-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O B-Citation I-Citation O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O O O O O O O O O O O O O B-Wavelength I-Wavelength O O B-CelestialObject O O O O O O O O O O B-CelestialObject O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation O O O O O O O O O O O O O O O O O O O O