Fraction of the donor star remaining at different evolutionary stages in the HMS-HMS grid for Case B mass transfer at 10⁻⁴ Z_⊙. The mass fraction of the total star before mass transfer (gray), after mass transfer (blue), and at core-carbon depletion (green) are shown for the binary models. The figure also includes the helium core fraction of the binary models at core-carbon depletion (red). The colored regions mark the possible ranges of the fractions for different initial periods and mass ratios of binaries undergoing stable Case B mass transfer. Additionally, we show the total mass (solid black) and helium core (dashed black) fraction at carbon-depletion of the single star models.