HR diagram showing the evolution of a system composed of a primary (the stripped star progenitor) with m_{stripped, i} = 3.3 M_⊙ and a secondary (the Be star progenitor) with m_Be, i = 1.7 M_⊙, in orbit with an initial period of P_i = 1.5 days. The system evolves via fully conservative MT. Dots along the tracks are separated by 0.1 Myr of evolution. Purple and blue highlighted regions correspond to mass transfer on the MS (case A) and after the MS (case AB), respectively. The scatter points mark the observed locations of HR6819 according to Bodensteiner et al. (2020), with 1σ error bars. An evolutionary track for a donor star (ZAMS mass 4.6 M_⊙) undergoing case B mass transfer is shown in blue as it crosses HR6819’s observable at its median. The ZAMS and stripped stars’ masses for every track are also reported along the track with corresponding colors.