Evolution of central chemical elements in binary systems with different initial orbital periods and mass ratios for both components. Panel (a): Central hydrogen evolution for systems with initial orbital periods ranging from 1.2 to 1.6 days. Panel (b): Central hydrogen evolution for systems with companion masses between 8.0 M_⊙ and 12 M_⊙. Panel (c): Same as panel (a) but for helium evolution. Panel (d):Same as panel (b) but for helium evolution. The “RMT” denotes reverse mass transfer from the initially less massive to the initially more massive star.