  Context: Brown dwarfs exhibit complex atmospheric signatures, and their
properties depend sensitively on effective temperature, surface gravity, and
metallicity. Several physical properties of brown dwarfs in binary systems can
be well inferred from the primary, and therefore allow to better constrain
their atmospheres.
  Aims: We want to constrain atmospheric models of brown dwarfs in binary
systems using narrow-band mid-infrared photometry.
  Methods: High spatial resolution and sensitivity is required to resolve the
components. Therefore we have obtained deep mid-infrared images of four close
binary systems with brown dwarf companions using VISIR at the VLT in three
narrow-band filters at 8.6, 10.5 and 11.25 micron.
  Results: Three brown dwarfs companions (GJ 229B, HD 130948BC and HR 7329B)
were detected at 8.6micron. HD 130948BC was also observed at 10.5micron. We
finally place upper flux limits for the other narrow band filters with null
detections.
  Conclusions: Our results are in general compatible with previous observations
and model expectations for these objects. For HD 130948BC, we conclude
photometric variability on a significance level of 2.8sigma based on repeated
observations. The bandpass around 10.5micron appears specifically well suited
for variability studies, and we speculate that either inhomogeneities in the
atmospheric NH3 distribution, or silicate absorption might cause its
time-variability.
