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With the discovery of Y dwarfs by the WISE mission, the population of fieldbrown dwarfs now extends to objects with temperatures comparable to those ofSolar System planets. To investigate the atmospheres of these newly identifiedbrown dwarfs, we have conducted a pilot study monitoring an initial sample ofthree late T-dwarfs (T6.5, T8 and T8.5) and one Y-dwarf (Y0) for infraredphotometric variability at multiple epochs. With J-band imaging, each targetwas observed for a period of 1.0h to 4.5h per epoch, which covers a significantfraction of the expected rotational period. These measurements represent thefirst photometric monitoring for these targets. For three of the four targets(2M1047, Ross 458C and WISE0458), multi-epoch monitoring was performed, withthe time span between epochs ranging from a few hours to ~2 years. During thefirst epoch, the T8.5 target WISE0458 exhibited variations with a remarkablemin-to-max amplitude of 13%, while the second epoch light curve taken ~2 yearslater did not note any variability to a 3% upper limit. With an effectivetemperature of ~600 K, WISE0458 is the coldest variable brown dwarf publishedto-date, and combined with its high and variable amplitude makes it afascinating target for detailed follow-up. The three remaining targets showedno significant variations, with a photometric precision between 0.8% and 20.0%,depending on the target brightness. Combining the new results with previousmulti-epoch observations of brown dwarfs with spectral types of T5 or later,the currently identified variables have locations on the colour-colour diagrambetter matched by theoretical models incorporating cloud opacities rather thancloud-free atmospheres. This preliminary result requires further study todetermine if there is a definitive link between variability among late-T dwarfsand their location on the colour-colour diagram.

The Brown-dwarf Atmosphere Monitoring (BAM) Project II: Multi-epoch monitoring of extremely cool brown dwarfs