Hertzprung–Russell diagrams of 6, 7, 8, and 9 M_⊙ stars of six different metallicities. The symbols indicate structures of individual Cepheid variable stars that lie in the instability strip (pink shaded area) and that we examine further in Appendix A. The blue and red edges of the instability strips were estimated from the work of Anderson et al. (2016) for z = 0.002, z = 0.006 and z = 0.014, from the work of Alibert et al. (1999) for z = 0.004, and from the theoretical relationships of Bono et al. (2000) for z = 0.008 and z = 0.020. L is the luminosity of the star, expressed in units of the luminosity of the sun L_⊙ (we use L_⊙ = 3.828 × 10³³erg.s⁻¹), and T_eff denotes its effective temperature. Evolutionary tracks with different masses are distinguished by color.