Each of four WR binaries has been observed in the 0.1 −5 keV range on four consecutive days for a total of ∼ 104 sec with the IPC on the Einstein Observatory. One of the stars is the well-known WN5 + 06 binary V444 Cyg; the other three have suspected compact companions; all have periods close to four days.
Neither HD 197406 (WN7) nor HD 96548 (WN8) was detected; hence Lx (0.5–3.0 keV) ≲ 1032 erg s−1, which is on the low side, even for 0-stars. EZ CMa (WN5) and V444 Cyg are moderately strong, variable X-ray sources with Lx (0.5–3.0 keV) ≃ 1033 erg s−1, kT ≃ 0.5 keV and NH ≃ 1022 cm−2. These values suggest that the X-ray flux arises in the outer part of the wind. The variable component may be produced by collision of two winds.
There is no evidence of harder X-rays coming from accretion onto a compact star. Nevertheless, the presence of a collapsar is not excluded at hv ≲ 5 keV, where absorption in the WR wind would likely reduce the X-ray flux to a level which is masked by the flux coming from the hot stellar wind.