Comparison of the blue wing of O Iλ8446 between the difference spectrum from 2015 September 24 to 2017 October 23 (blue) and the host-subtracted residuum spectrum from 2015 September 24 (slightly smoothed and scaled for clarity; red) resulting from the stellar synthesis in Sect. 3.1.1. The shift of the blue wing of O Iλ8446 from 2015 September 24 to 2017 October 23 indicates a blueward drift of the line emission. The difference velocity Δv_shift amounts to Δv_shift ≈ 150 − 200 km s⁻¹.