Location of the AT 2021blu progenitor in the HRD (cyan starred symbol), and comparison with different solar-metallicity evolutionary tracks. Top-left panel: comparison with evolutionary tracks for single stars. The track of a 14 M_⊙ star is indicated with a blue solid line, those of 13 and 15 M_⊙ with dot-dashed lines, all other tracks with blue dotted lines. Bottom-left panel: comparison with tracks for binary systems with initial orbital periods P ≈ 100 days (log(P) = 2). Top-right panel: comparison with tracks for binary systems with initial orbital periods P ≈ 40 days (log(P) = 1.6). Bottom-right panel: comparison with tracks for binary systems with initial orbital periods P ≈ 15 days (log(P) = 1.2). Binary tracks with primaries of 12–20 M_⊙ are shown as lines with different colours. Binary tracks for different mass ratios (q = M₁/M₂, where M₁ indicates the mass of the primary star and M₂ that of the secondary star) are also shown. All tracks are taken from the BPASS database (Eldridge et al. 2017; Stanway & Eldridge 2018). The insets show a close-up view of the location of the AT 2021blu progenitor in the HRD, along with the tracks of different configurations of binaries of 13–18 M_⊙ that intersect the error bars of the AT 2021blu progenitor.