Hertzsprung-Russell diagram showing stellar evolution models evolving across the Hertzsprung gap and the first crossing of the Cepheid instability strip. Solid lines follow evolution models with initial mass M₁ = 3.8 M_⊙ (gray), along with stellar merger models with progenitor initial masses 2.1 (violet), 2.2 (orange), 2.3 (blue), 2.4 (pink) and 2.5 M_⊙ (brown). The blue region represents the measured Cepheid radius while the dashed lines denote the blue edge of the first-overtone instability strip “1OTBE” (dark blue), the boundary between first-overtone and fundamental-mode instability strips “FMBE” (light blue) and the red edge of the Cepheid instability strip “RE” (red) (Bono et al. 2000).