Using a set of high-resolution spectra , we studied the physical and orbital properties of the O-type binary CPD - 41 ^ { \circ } 7733 , located in the core of NGC 6231 .
We report the unambiguous detection of the secondary spectral signature and we derive the first SB2 orbital solution of the system .
The period is 5.6815 \pm 0.0015 d and the orbit has no significant eccentricity .
CPD - 41 ^ { \circ } 7733 probably consists of stars of spectral types O8.5 and B3 .
As for other objects in the cluster , we observe discrepant luminosity classifications while using spectroscopic or brightness criteria .
Still , the present analysis suggests that both components display physical parameters close to those of typical O8.5 and B3 dwarfs .
We also analyze the X-ray light curves and spectra obtained during six 30 ks xmm -Newton pointings spread over the 5.7 d period .
We find no significant variability between the different pointings , nor within the individual observations .
The CPD - 41 ^ { \circ } 7733 X-ray spectrum is well reproduced by a three-temperature thermal mekal model with temperatures of 0.3 , 0.8 and 2.4 keV .
No X-ray overluminosity , resulting e.g .
from a possible wind interaction , is observed .
The emission of CPD - 41 ^ { \circ } 7733 is thus very representative of typical O-type star X-ray emission .