Notes. Summary of X-ray detections by spectral type. The top panel reflects the expected evolutionary pathway for single stars, the second the possible products of binary interaction and the third WRs – a structure chosen to reflect the discussion in Sect. 3. We note that the marginal X-ray detections discussed in Sect. 3.4 are not included here. The X-ray detections Wd1-13, W1033 and Wd1-232 are included with the B0-2Ia⁺/WNVL, O9-9.5 II-III and B0-0.5 Ia,Iab,Ib sub-sets, respectively. The OB2 SB2 grouping comprises the optically and X-ray luminous systems Wd1-36 and -53a. A further ten optically faint O+O binaries of uncertain spectral type and luminosity class are excluded from the table, as is the sole Be star; none of these are X-ray detections.