Hypothetical scheme of evolution of the Hα line profile when the main shock wave propagates in the envelope of an RR Lyrae star. The early stage occurs just before the luminosity maximum (pulsation phase 0.9), i.e., when the shock front is close to the photosphere. During the late stage (pulsation phase around 0.3), a P-Cygni profile is observed. It means that the shock front is detached from the photosphere. The expanding shell is actually the radiative wake of the shock in which the Hα  emission line is formed. λ_e is the wavelength of the maximum intensity of the Hα  emission, λ_a is the wavelength of the minimum intensity of the Hα  absorption and λ₀ the laboratory wavelength.