  We present photographic B, R and I photometry, and optical and near-infrared
spectroscopy, of a new ultracool white dwarf (UCWD) discovered in the
SuperCOSMOS Sky Survey. The spectrum of SSSJ1556-0806 shows strong flux
suppression due to the presence of collisionally induced absorption by
molecular hydrogen (H2CIA), a feature characteristic of the cool, high density
environments found in the atmospheres of ultracool white dwarfs. SSSJ1556-0806
therefore joins a list of <10 ultracool white dwarfs displaying extreme flux
suppression. Synthetic model fitting suggests an effective temperature <3000K,
which if true would make this one of the coolest white dwarfs currently known.
We also exploit the similarity between the SEDs of SSSJ1556-0806 and the
well-studied UCWD LHS 3250 to aid in the determination of the atmospheric
parameters in a regime where models consistently fail to reproduce
observations. SSSJ1556-0806 is relatively bright (R ~ 17.8), making it
particularly amenable to follow up observations to obtain trigonometric
parallax and IR photometry.
