  The characterization of multiple stellar systems is an important ingredient
for testing current star formation models. Stars are more often found in
multiple systems, the more massive they are. A complete knowledge of the
multiplicity of high-mass stars over the full range of orbit separations is
thus essential to understand their still debated formation process.
Observations of the Orion Nebula Cluster can help to answer the question about
the origin and evolution of multiple stars. Earlier studies provide a good
knowledge about the multiplicity of the stars at very small (spectroscopic) and
large separations (AO, speckle) and thus make the ONC a good target for such a
project. We used the NIR interferometric instrument AMBER at VLTI to observe a
sample of bright stars in the ONC. We complement our data set by archival NACO
observations of \theta 1 Ori A to obtain more information about the orbit of
the close visual companion. Our observations resolve the known multiple systems
\theta 1 Ori C and \theta 1 Ori A and provide new orbit points, which confirm
the predicted orbit and the determined stellar parameters for \theta 1 Ori C.
Combining AMBER and NACO data for \theta 1 Ori A we were able to follow the
motion of the companion from 2003 to 2011. We furthermore find hints for a
companion around \theta 1 Ori D and a previously unknown companion to NU Ori.
With a probability of ~90% we can exclude further companions with masses of > 3
Msun around our sample stars for separations between ~2 mas and ~110 mas. We
conclude that the companion around \theta 1 Ori A is most likely physically
related to the primary star. The newly discovered possible companions further
increase the multiplicity in the ONC. For our sample of two O and three B-type
stars we find on average 2.5 known companions per primary, which is around five
times more than for low-mass stars.
