Astron. Astrophys. 323, 853-875 (1997)

7. Conclusions

Using the cross-correlation in position of the OB star catalogues held in SIMBAD with the low galactic latitude part of the RASS we selected 24 early type stars which apparently exhibited a soft X-ray excess over the normal stellar level. Follow-up optical and X-ray observations allowed to strongly suggest the presence of a compact object, probably a neutron star, in four cases and establish the X-ray binary nature of LS I +61 235. In two additional cases which still require confirmation the X-ray luminosity may be compatible with that expected from an accreting white dwarf.

The new massive X-ray binary candidates are clearly representative of the low end of the X-ray luminosity function. Their soft X-ray luminosities are comparable to those of hard X-ray transients in quiescence and those of the low luminosity Be/X-ray systems such as X Persei. One of the new Be/X-ray systems is the probable counterpart of 4U 1036-56 which was in outburst during the years 1970-1976.

Four B stars located in the Orion and Canis Major OB associations exhibit X-ray luminosities in the range of 2-7 10³¹ erg s^-1 well above the expected normal stellar emission. Such X-ray luminosities appear at the high end of the luminosity distribution of active stars and the usual interpretation in terms of a young late type companion star may not hold.

Finally we report the probable discovery of a white dwarf companion to the B5V type star HR 2875.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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