Target regions of the parameter space for GW151226 (square markers) and GW170608 (cross markers) for models with α_CE = 2.0. Left panels: progenitor initial masses (M_i, 1 > M_i, 2). Right panels: merger time delay (t_merge) against initial binary separation (a_i). Panels from top to bottom correspond to each set of efficiencies: ϵ = 0.6, 0.4, 0.2. Dashed lines indicate equal progenitor masses.