Detection of <i>J</i> -band photometric periodicity in the T8 dwarfs 2MASS J09393548-2448279 and EQ J1959-3338

Aims. We aim to study the near-infrared variability of the T8 dwarfs 2MASS J09393548-2448279 and EQ J1959-3338 by analyzing their J -band photometric signal, which can provide new insights into the atmospheric dynamics of cold brown dwarfs. Methods. We used FLAMINGOS-2 on the Gemini South telescope to perform J -band differential photometry continuously over 4 h for each target. The resulting light curves have a cadence of 20 s and a photometric uncertainty of 2–4 mmag. Results. We detect periodic variability in both T8 dwarfs, with an amplitude of 16.6 ± 0.9 mmag and a period of 1.364 ± 0.012 h for EQ J1959-3338, which we attribute to rotational modulation. For 2MASS J09393548-2448279, we observe an amplitude of 4.6 ± 0.4 mmag and a period of 1.733 ± 0.040 h, though this periodicity could represent a fraction of a longer period. Conclusions. With the detection of variability in 2MASS J09393548-2448279 and EQ J1959-3338, the number of known variable T8 dwarfs has doubled, making them prime candidates for infrared space-based monitoring and radio observations to investigate atmospheric dynamics and the influence of the magnetic field in very cool atmospheres.