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Abstract

In this work I aim characterize a number of (sub)-stellar companions. When studying isolated brown dwarfs, very low-mass stars, and directly imaged exoplanets with insignificant orbital motion, we have to rely on theoretical models to determine basic parameters such as mass, age, and consequently, effective temperature and surface gravity. While stellar and atmospheric models are rapidly evolving, we need a powerful tool to test and calibrate them. One way to test models is to compare theoretical isochrones produced by interior and atmospheric models with observed sequences in open clusters. Open clusters contain many objects of the same chemical composition and age, and spanning a range of masses. Using our own Lucky imaging observations and literature data, I constructed a single-star sequence for the Hyades open cluster. I compare the obtained sequence to a set of theoretical isochrones identifying systematic offsets and revealing probable issues in the models. However, there are many cases when it is impossible to test models before applying them to observations. One example is applying atmospheric models for constraining parameters of the coolest known Y dwarf WISE 0855-07. I demonstrate the limits of constraining effective temperature and the presence/absence of water clouds that are introduced by unknown systematic effects in models and observations. In the final chapter I introduce a novel method to take into account the above-mentioned systematics. I construct a ”systematics vector” that allows us to reveal problematic wavelength ranges when fitting atmospheric models to observed near-infrared spectra of brown dwarfs and directly imaged exoplanets. This approach plays a crucial role when retrieving abundances of brown dwarfs and exoplanets, in particularly, a C/O ratio. The latter parameter is an important key to formation scenarios of brown dwarf and exoplanets. I show the way of constraining this parameter while eliminating systematics effects, which significantly improves the reliability of a final result and our conclusions about formation history of certain exoplanets and brown dwarfs.