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

Temperature constraints on the coldest brown dwarf known WISE 0855-0714