Profile of the binding energy parameter λ _CE as a function of mass coordinate for accretors (orange), single stars (red), and our engineered stars (other colors) at selected total radii. The vertical lines mark the outer edge of the helium cores of the accretor and single star, which is the outermost location where Y > 0.1 and X < 0.01 (solid lines), or X < 0.1 (dashed), or X < 0.2 (dotted–dashed). The CEB size of engineered models increases from blue to yellow.