Same colour scheme as in Fig. 7. Zoom on the 3.4 µm hydrocarbon feature seen in polarised extinction. The dashed lines show the upper limits measured by Chiar & Tielens (2006) towards GCS 3-II and GCS 3-IV, lowest and highest lines, respectively, above the continuum of our four best-fit models (see Sect. 6.3 for details). For comparison, we also show in grey the model of Hensley & Draine (2023) which complies with the upper limit measured towards GCS 3-IV but not with the lower one towards GCS 3-II. Hensley & Draine (2023) gives indeed ∆p_{3.4 µm} 2 to 2.6 times as large as our best models.