  Observational data on companion statistics around young stellar systems is
needed to flesh out the formation pathways for extrasolar planets and brown
dwarfs. Aperture masking is a new technique that is able to address an
important part of this discovery space. We observed the two debris disk systems
HD 92945 and HD 141569 with sparse aperture masking (SAM), a new mode offered
on the NaCo instrument at the VLT. A search for faint companions was performed
using a detection strategy based on the analysis of closure phases recovered
from interferograms recorded on the Conica camera. Our results demonstrate that
SAM is a very competitive mode in the field of companion detection. We obtained
5 sigma high-contrast detection limits at lambda/D of 2.5x10^{-3} (\Delta L' =
6.5) for HD 92945 and 4.6x10^{-3} (\Delta L' = 5.8) for HD 141569. According to
brown dwarf evolutionary models, our data impose an upper mass boundary for any
companion for the two stars to, respectively, 18 and 22 Jupiter masses at
minimum separations of 1.5 and 7 AU. The detection limits is mostly independent
of angular separation, until reaching the diffraction limit of the telescope.
We have placed upper limits on the existence of companions to our target
systems that fall close to the planetary mass regime. This demonstrates the
potential for SAM mode to contribute to studies of faint companions. We
furthermore show that the final dynamic range obtained is directly proportional
to the error on the closure phase measurement. At the present performance
levels of 0.28 degree closure phase error, SAM is among the most competitive
techniques for recovering companions at scales of one to several times the
diffraction limit of the telescope. Further improvements to the detection
threshold can be expected with more accurate phase calibration.
