For the BSE models: distribution of the estimated X-ray luminosity, L_X, with the spin period of the stripped He star during the He+BH phase (sampled every 0.02 Myr), for metallicities Z = 0.01 Z_⊙, and Z_⊙ and two criteria for the mass transfer stability (see text). All He+BH systems with orbital periods greater than 1 d that merge within a Hubble time end up in yellow (see the colour bar).