  We present adaptive optics JHKs imaging observations of three main-sequence
late B-type stars listed in the Lindroos Catalogue: HD123445, HD127971 and
HD129791. Given their spectral types, these stars should not be X-ray emitters.
However, they have been detected by ROSAT and their X-ray emission has been
attributed to possible unresolved late-type companions. We have carried out
near-IR observations with ADONIS at the ESO 3.6m but have not detected any
late-type companions close to HD127971 and HD129791. This result leads us to
conclude that either (i) they are spectroscopic binaries with unresolved
low-mass companions, or (ii) they are intrinsic X-ray emitters. While the
former case would be consistent with the reported high multiplicity of
early-type (A and B) stars, the latter would yield a revision of stellar
activity theories which do not predict X-ray emission from these stars. On the
other hand, HD123445 does indeed show visual companions, namely an apparent
subarcsecond faint (Ks=10mag) binary system at a projected separation of 5"
from the late-B type star. The JHKs magnitudes and colors of the components are
consistent with a pair of Pre Main Sequence (PMS) K-type stars at 140 pc, i.e.
possible members of the Upper Centaurus Lupus association. In this case the
reported X-ray emission can be ascribed to the new binary system. Nevertheless,
spectroscopy is required to confirm the possible PMS nature of the pair.
