Program: GN-2021B-Q-326

We propose to continue a successful monitoring program to discover new photometrically variable late-T type brown dwarfs. Brown dwarfs at the T/Y transition are predicted to have clouds of sulfide, salt, or water. If the cloud cover is patchy, these clouds can produce rotationally-modulated photometric variability, as in warmer objects dominated by silicate clouds. However, these cold faint objects have been under-represented in variability studies, which have focussed mainly on (brighter) L/T-transition objects. Detecting the signatures of the more volatile clouds expected at the ~500 K T/Y transition will bring us closer to understanding the cloud dynamics in atmospheres with temperatures similar to Earth's. From a limited 2018B-2019A pilot study we discovered two late-T variables, and so doubled the number of known <700 K cloudy brown dwarfs. With our ongoing 2021A program and with the proposed 2021B observations we aim to discover up to four more late-T variables. While our time request is sizeable, our program is suitable for Band 3 queue-mode execution.