Impact of X-ray radiation on the C IV 5801-12 lines, for a supergiant model with T_eff = 40 kK (S40). Black: line profiles for the “standard” model without shocks; green: model with an intermediate X-ray strength; red: model with strong X-ray radiation. The dotted profiles refer to alternative, no-shock models with reduced carbon abundance, mimicking the effect of shock radiation. We note that the displayed sensitivity is strongest for the considered lines and parameter range, see text.