  Context: L-type ultra-cool dwarfs and brown dwarfs have cloudy atmospheres
that could host weather-like phenomena. The detection of photometric or
spectral variability would provide insight into unresolved atmospheric
heterogeneities, such as holes in a global cloud deck.
  Aims: It has been proposed that growth of heterogeneities in the global cloud
deck may account for the L- to T-type transition as brown dwarf photospheres
evolve from cloudy to clear conditions. Such a mechanism is compatible with
variability. We searched for variability in the spectra of five L6 to T6 brown
dwarfs in order to test this hypothesis.
  Methods: We obtained spectroscopic time series using VLT/ISAAC, over
0.99-1.13um, and IRTF/SpeX for two of our targets, in J, H and K bands. We
search for statistically variable lines and correlation between those.
  Results: High spectral-frequency variations are seen in some objects, but
these detections are marginal and need to be confirmed. We find no evidence for
large amplitude variations in spectral morphology and we place firm upper
limits of 2 to 3% on broad-band variability, on the time scale of a few hours.
The T2 transition brown dwarf SDSS J1254-0122 shows numerous variable features,
but a secure variability diagnosis would require further observations.
  Conclusions: Assuming that any variability arises from the rotation of
patterns of large-scale clear and cloudy regions across the surface, we find
that the typical physical scale of cloud cover disruption should be smaller
than 5-8% of the disk area for four of our targets. The possible variations
seen in SDSS J1254-0122 are not strong enough to allow us to confirm the cloud
breaking hypothesis.
