Hertzsprung–Russell diagram showing the evolution of the binary. Tracks are coloured by the mass loss and mass accretion rate of the donor (marked D1-5) and accretor (marked A1-4); see Sect. 3.1 for an explanation of the labels. We limited the accretion on the companion to 0.5 M_⊙ and followed its further evolution until core hydrogen exhaustion (see Sect. 2.1 for details on the technical implementation). The point at which 0.5 M_⊙ of material has been accreted is noted with a grey circle and subsequent evolution is highlighted in grey. For reference, we show single star tracks as dashed light grey curves, with masses from 3 to 4 M_⊙ in 0.1 M_⊙ intervals. An interactive plot is available at https://www.tomwagg.com/html/interact/mass-gainer-asteroseismology.html#fig1.