Density profiles at the moment of core collapse for single (blue) and binary-stripped star models (red) with similar reference core masses. The mean molecular weight profiles are indicated with dashed lines. From left to right, shaded regions give the approximate locations of the iron-rich, oxygen-rich, helium-rich, and hydrogen-rich regions, labeled with Roman numerals as in Fig. 2. The leftmost models, with a reference core mass of 4.3 M_⊙, both experience shell mergers during their evolution. The density of the binary-stripped star models in the helium-rich region (I) is systematically higher than that of the single-star models with a similar reference core mass.