star was actually $\sim$15% cooler than model fitting would have predicted and @Dupuy.2009 and @Liu.2008 reached similar conclusions when studying brown dwarf binaries with dynamical masses.

Under those circumstances, observations are key to the development of the models. Only five brown dwarfs with temperatures below 650K (T8.5) are currently known from recent discoveries by UKIDSS and CFBDS, and this small number prevents discerning general trends from individual peculiarities. @Kirkpatrick.1999 and @Burgasser.2002 could rely on samples of 20-25 objects to respectively define the L and T spectral types. In this article we present the CFBDSIR, a near infrared (hereafter NIR) extension to the CFBDS that will provide a WIRCAM [@Puget.2004] $J$-band coverage down to $J_{vega}=20.0$ for 10$\sigma$ detections atop 335 square degrees of CFBDS MegaCam [@Boulade.2003proc] $z'$-band imaging with a 5$\sigma$ detection limit of $z'_{AB}$=23.25-24.05. All optical magnitudes presented in this article are AB magnitudes, while all NIR magnitudes are Vega magnitudes. When the CFBDSIR is complete, we hope to achieve a threefold increase in sample size to 15-20 characterised ultracool brown dwarfs and possibly find a few substellar objects significantly cooler than 500K of which none is known yet outside the solar system. This will define general trends and dispersions 