HRD with evolutionary tracks for a merger product of M_⋆ = 6.0 + 2.4 M_⊙ and a genuine single star with an initial mass of M_⋆ = 7.8 M_⊙ (Panel a), and a merger product of M_⋆ = 9.0 + 6.3 M_⊙ and a genuine single star with an initial mass of M_⋆ = 13.6 M_⊙ (Panel b). The merger and genuine single-star tracks are shown in black and grey, respectively. To avoid cluttering, the track of the merger model is not shown during the fast accretion phase. A dashed black line replaces it. Each evolutionary track is connected to its label through a symbol on the HRD at the ZAMS. The green plus symbols indicate the position in the HRD after 10⁶ years have passed since the merging or TAMS for the merger product and single star, respectively. The colours on the tracks are related to the buoyancy travel time, Π₀. The violet box indicates the position at which we compare the asteroseismic properties of the merger product and genuine single star in Sects. 4.1.1 and 4.1.2.