| Abstract
| - Aims. The aim of the project is to contribute to the characterisation of the spectral energy distribution of the coolest brown dwarfs discovered to date, the Y dwarfs. Methods. We obtained z-band far-red imaging for six Y dwarfs and a T9 + Y0 binary with the OSIRIS (Optical System for Imaging and low Resolution Integrated Spectroscopy) instrument on the 10.4-m Gran Telescopio de Canarias (GTC). Results. We detect five of the seven known Y dwarfs in the z-band, infer their optical-to-infrared colours, and measure their proper motions. We find a higher dispersion in the z − J and z − H colours of Y0 dwarfs than in T dwarfs. This dispersion is found to be correlated with H − w2. The high dispersion in the optical-to-infrared colours of Y dwarfs and the possible turnover towards bluer colours may be a consequence of the presence of sulfide clouds with different thicknesses, the depletion of alcalines, and/or gravity effects.
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