Histogram of the Monte Carlo DNS sampling q-factor that is defined as the mass ratio of a recycled NS to its companion NS. In the process of Monte Carlo sampling, the mass of the first-formed recycled NS obeys a double-Gaussian distribution, in the range of 1.25−1.62 M_⊙, while the mass of the second-formed non-recycled NS (companion) prefers a single-Gaussian distribution, in the range of 1.17–1.46 M_⊙. The masses of 100 000 pairs of DNSs ware randomly selected based on the above selection conditions.