There is a recent observational evidence that some young very low mass objects exhibit a strong photometric variability. The origin of this variability is mysterious and various scenarios such as an ongoing magnetospheric accretion or eclipses by planetary companions are possible. The aim of this proposal is to search for any associated spectroscopic variability of these objects. This will enable us to classify the objects, confirm/refute their brown dwarf status, determine their basic properties (temperatures, radii, masses, ages), distinguish between various scenarios, explore other alternatives, and ultimately reveal the true nature of this variability. The project will verify if the magnetospheric accretion works in such objects and put the constraints on the duration of the accretion phase. Alternatively, it may lead to a discovery of unique low mass binary systems or transiting extrasolar planets. The researcher also aims to perform computer simulations and study the synthetic light-curves and spectra of accreting brown dwarfs as well as the synthetic lightcurves, spectra and transit radius spectra of the extrasolar planets. It is hoped that, by comparing these simulations with the existing and future observations, new species will be identified in the atmospheres of the extrasolar planets. For this purpose, the researcher will create a new version of the computer code SHELLSPEC and will make it available to the astronomical community on the web with the complete documentation.