Specific entropy s (top row), H (bottom row, solid lines), and He (bottom row, dashed lines) TAMS profiles for nonrotating single stars (red), accretors (orange), and “engineered” models of the same total mass as the post-RLOF mass of the accretors. The overlapping gray bands emphasize the CEB region, X _c(¹H) + 0.01 < X(¹H) < X _surf(¹H) − 0.01, with X _c and X _surf the central and surface value of the hydrogen mass fraction. The CEB size of the engineered models increases from blue to yellow.⏬ ✎