Zoomed-in part of the top panel (Model 1) of Fig. 3 (described in Sec. 7.2 and Sec. 7.3) where the points cluster the most. Blue crosses correspond to XRBs that form merging DCOs and open symbols show the rest of the XRB synthetic population in Model 1 (see Sec. 6.1). The figure shows that there are no XRBs that form merging DCOs with M_donor < 2.0 M_⊙ and P_orb < 2.0 days. We note that the merging DCO progenitors are ten times over-represented with respect to other XRBs on the plot to show which part of the parameter space they occupy.