New constraints on the multiplicity of massive young stars in Upper Scorpius

Context. Observations and simulations have clearly established that most stars form in multiple systems. Characterizing their properties is thus important for our understanding of the star formation process. Aims. To provide statistics about the number of companions per star over the full range of angular distances, infrared long-baseline interferometric studies can be employed to fill the gap between spectroscopic and adaptive optics searches. The Upper Scorpius OB association is a good target for such observations, because its stellar content is very well known from both spectroscopic and adaptive optics searches. Methods. We used the ESO Very Large Telescope Interferometer to perform long-baseline interferometric observations of a sample of seven B stars. Furthermore, we used ROSAT X-ray data to search for indications of low-mass companions. Results. With the interferometric observations, we find previously known companions around sigma Sco and HR 6027. For the other targets we determine the parameter space in which the presence of companions can be excluded from our data. For two of the B stars in our sample, pi Sco and HR 6026, the detection of X-ray emission provides indirect evidence of previously unknown low-mass companions. Conclusions. In total we find two previously unknown companions. We can exclude the presence of other unknown companions within the separation range of similar to 2 to similar to 100 mas and for a brightness ratio >= 0.1.