Evolution of central chemical elements in binary systems with different initial orbital periods and component mass ratios. Panel (a): Central oxygen evolution for systems with orbital periods of 1.2–1.6 days. Panel (b): Central oxygen evolution for systems with companion masses of 8.0 M_⊙ to 12 M_⊙. Panel (c): Same as panel (a) but for carbon evolution. Panel (d): Same as panel (b) but for carbon evolution.