Evolution in the Hertzsprung-Russell diagram of the primary (top) and secondary (bottom) stars in binary systems with Z = 10^-3.5 ≃ Z_⊙/ 50 consisting of a 70 M_⊙ primary with mass ratios q = 0.2,0.6 and initial orbital periods that are close to producing RLOF at the ZAMS. The dotted line shows the location of the ZAMS for non-rotating stars; the track of a non-rotating 70 M_⊙ star is also shown for reference. The system with initial mass ratio q = 0.6 has the primary evolving chemically homogeneously, but the secondary initiates mass transfer before a BH is formed. The system with initial q = 0.2 manages to form a BH and afterwards undergoes three distinct phases of mass transfer. See Sect. 3.1 for details.