  Context. Nearby isolated planetary mass objects are beginning to be
discovered, but their individual properties are poorly constrained because
their low surface temperatures and strong molecular self-absorption make them
extremely faint.
  Aims. We aimed to detect the near infrared emission of the coldest brown
dwarf (BD) found so far, WISE0855$-$0714, located $\sim$2.2 pc away, and to
improve its temperature estimate (T$_{\rm eff}$= 225-260 K) from a comparison
with state-of-the-art models of BD atmospheres.
  Methods. We observed the field containing WISE0855-0714 with HAWK-I at the
VLT in the $Y$ band. For BDs with T$_{\rm eff}<$500\,K theoretical models
predict strong signal (or rather less molecular absorption) in this band.
  Results. WISE0855-0714 was not detected in our Y-band images, thus placing an
upper limit on its brightness to Y>24.4 mag at 3-$\sigma$ level, leading to
Y-[4.5]>10.5. Combining this limit with previous detections and upper limits at
other wavelengths, WISE0855$-$0714 is confirmed as the reddest BD detected,
further supporting its status as the coldest known brown dwarf. We applied
spectral energy distribution fitting with collections of models from two
independent groups for extremely cool BD atmospheres leading to an effective
temperature of T$_{\rm eff}<$250\,K,.
