A search for fast photometric variability in very low mass stars and brown dwarfs Samrat Ghosh 1* , Soumen Mondal 1 , Ramkrishna Das 1 , Santosh Joshi 2 , Sneh Lata 2 , Siddhartha Biswas 1 1 S. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata 700106, India 2 Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital-263002, India Abstract: We present here I-band time-series photometric studies of a few very low-mass stars (VLMs) and brown dwarfs (BDs) in the young (age of 2-3 Myr) IC348 star-forming region in the Perseus Molecular Cloud. Our main aim is to explore the fast rotating (of time scale of a few hours) BDs and VLMs in the I-band. From our preliminary results, we find significant variability in the young M5 dwarf 2MASS J03443896+3203196 with a rotation period of 16.037 hours. Keywords: low-mass star – brown dwarfs – photometry – variability 1 Introduction Very Low-Mass stars (VLMs) are main-sequence stars with a spectral type from mid-K to late-M and a mass from about 0.6 M ⊙ down to the brown dwarf limit of 0.075 M ⊙ (Allard et al. 1997), while Brown Dwarfs (BDs) occupy the gap between the least massive stars and the most massive planets with a theoretical mass range of 13 - 75 M J with solar metallicity. Due to their low mass, their core pressure does not reach the threshold for sustaining hydrogen burning, instead, these substellar objects burn deuterium. Brown Dwarfs were first discovered in 1995 (Nakajima et al. 1995), and after that more than thousand Brown Dwarfs were discovered in our galaxy by ground- as well as space-based optical/IR all-sky surveys in various star-forming regions as well as in the field. Low temperatures of such L/T dwarfs allow molecules to condense and form dust clouds. The majority of these VLMs and BDs are found to be photometrically variable with rapid rotation (Herbst et al. 2000; Rockenfeller et al. 2006; Zapatero Osorio et al. 2006; Crossfield 2014). Vari- ability in these dwarfs is attributed due to the presence of surface features like magnetic spots (due to strong magnetic fields) or dust clouds, which cause optical modulation as these objects rotate, and it is possible to measure the period of rotation of an object from its light curve (Radigan 2014; Metchev et al. 2015 and references therein). Their periodic variability is caused by rotational modulation of the stellar flux generated by an asymmetric distribution of cool spots or spot groups on the stellar surface. These cool spots form mainly due to the presence of dust clouds. This can happen in various * samrat687@gmail.com ”2nd Belgo-Indian Network for Astronomy & astrophysics (BINA) workshop”, held in Brussels (Belgium), 9-12 October 2018 Bulletin de la Société Royale des Sciences de Liège, Vol. 88, Actes de Colloques, 2019, p. 275 - 278