C ivλλ 1548.2, 1550.8 wind profile in the UV spectrum of SSN 31 (O7.5 Vz). Synthetic spectrum in red, observations in black. Top panel: synthetic spectrum with wind parameters v_∞ = 1000 km s⁻¹, β = 0.8, D = 10, and . See details in Sect. 4.1. Without ISM line models applied. Bottom panel: the same synthetic but with the addition of ISM lines C iv λλ 1548.2, 1550.8 doublet, and Si ii λ 1526.7 with the ISM properties detailed in Sect. 3.2.