1 Introduction

Binary properties of newly formed stars in various aggregates and nearby star-forming regions have been subjected to much observing efforts in the recent years, but most of them were devoted to the study of solar-type stars. The results have been summarized by Duchêne (1999) and Mathieu et al. (1999). The binary frequency appears to be larger in loose aggregates, like Taurus-Auriga, than in high density regions, like the Orion Trapezium. However, most regions studied offer little information on the duplicity of massive stars.

Surveys of O-type stars (Gies 1987; Mason et al. 1998) have shown that the binary frequency and multiplicity are higher in open clusters and associations than in the field. If one takes also into account spectroscopic studies, we have strong observational evidences that O-type stars in open clusters are very frequently found in binary or multiple systems.

There are however few clusters close enough to allow a deep investigation of the duplicity along the main sequence from O- to solar-type objects. NGC 6231 ( $\alpha_{2000}$ = 16$^{\rm h}$54 $\hbox{$.\!\!^{\rm m}$ }$2, $\delta_{2000}$ = -41 $\hbox {$^\circ $ }$48 $\hbox{$^\prime$ }$), in the southern part of the association Sco OB1, is certainly one of the best clusters to study the results of a fully completed star-formation process. It contains a dozen O9 stars (Garrison & Schild 1979; Levato & Malaroda 1980), many of them being double-lined binaries. The first radial velocities for 20 stars were obtained by Struve (1944), who detected six spectroscopic binaries. Orbits for 6 of them, with surprisingly short periods, had been already determined in 1974 by Hill et al. (1974), hereafter referred to as HCB. Levato & Morrell (1983), hereafter referred to as LM83, obtained further radial velocities and presented new orbital elements for 6 systems. Perry et al. (1990), hereafter referred to as PHYB, obtained 3 observations at 120 Å mm^-1 for stars brighter than V = 10.2. Stickland et al. (1996, 1997) published orbital elements for two stars based on IUE spectra. Finally, Raboud (1996) investigated 53 B-type stars and also concluded that the binary frequency is larger than 50%.

NGC 6231 seems to be one of the best candidate to study directly the global products of cloud-fragmentation and star-formation processes along the main sequence in spite of its distance, 1800 pc (Raboud et al. 1997). On the basis of photometric CCD observations, Sung et al. (1998) and Baume et al. (1998) examined the evidence for the presence of a population of pre-main-sequence stars, which should exist due to the cluster young age, and determined the initial mass function. The long-term goal of this study is to provide reliable data to define the characteristics of the duplicity of the O- and B-type stars in a very young open cluster just emerged from its nebulosity, namely the duplicity rate, the orbital-period- and $V\sin i$ distributions to constrain models of cluster and (binary) star formation.