Shock wave velocity estimated from Eq. (2) at different Blazhko phases. The red dots refer to the 90 s and the blue dots refer to the years 2013 and 2015. The uncertainty on the shock velocity induced by using Eq. (2) is certainly not smaller than 10 km s⁻¹ when the shock velocity varies respectively from 100 to 150 km s⁻¹. The uncertainty on the Blazhko phase is estimated at 0.1. The asymmetric vertical bar through each point represents the range of plausible values of V_s used to find the best fit. The straight linear regression lines through the points between 0.0 ⩽ ψ ⩽ 0.5 and 0.5 ⩽ ψ ⩽ 1.0 are shown. Their correlation coefficients R are 0.63 and 0.50, respectively.