Example of a grid of binary systems showing initial periods (in days) and initial primary masses for a fixed initial mass ratio of q = 0.8. Each rectangle represents a single detailed binary evolution model. Light blue is used for binaries with a primary collapsing into a NS while having a companion mass within the values derived by Kaper et al. (2006) (named ‘in range’). In a darker blue colour, we show binaries with primaries forming BHs or being outside the constraint imposed by the observed masses (named ‘out of range’). The hatched region represents a binary that ends its evolution without reaching a core-collapse stage. For the entire grid of models computed, we find that these non-collapsing models either merge during a CE phase between two non-degenerate stars, or stop due to numerical issues. Other initial mass ratios produce similar results.