Hertzsprung-Russell (H-R) diagram showing the evolutionary tracks of the primary stars (i.e. He stars) of He star + MS systems in our detailed stellar evolution calculations using the STARS code. Different curves show the evolution of the He star with masses of 0.70 M_⊙, 0.75 M_⊙, 0.80 M_⊙, and 0.85 M_⊙. The starting point of our calculations is given by black dots. The grey dashed lines represent constant radius lines on the H-R diagram.