Constraints on the initial progenitor separation, given binary constituent masses. As can be seen, a heavier system will inspiral faster, leading to a weaker constraint on the initial separation of the binary. The most probable value, the 68% and 98% posterior predictive intervals of the NS binary mass distribution from Kiziltan et al. (2013), are shown with the dashed ellipses, which corresponds to a constraint on a maximum initial separation of R_⊙, respectively.