Aims: Our science goals are to characterise the optical properties of Y dwarfs and to study their consistency with theoretical models. Methods: A sample of five Y dwarfs was observed with three optical and near-infrared instruments at the 10.4m Gran Telescopio Canarias. Deep near-infrared (J- or H-band) and multicolour optical images (z-, i-, r-, G-, u-bands) of the five targets and a low-resolution far-red optical spectrum for one of the targets were obtained.
Results: One of the Y dwarfs, WISE J173835.53+273258.9 (Y0), was clearly detected in the optical (z- and i-bands) and another, WISE J182831.08+265037.7 (Y2), was detected only in the z-band. We measured the colours of our targets and found that the z - J and i - z colours of the Y dwarfs are bluer than those of mid- and late-T dwarfs. This optical blueing has been predicted by models, but our data indicates that it is sharper and happens at temperatures about 150 K warmer than expected. The culprit is the K I resonance doublet, which weakens more abruptly in the T- to Y-type transition than expected. Moreover, we show that the alkali resonance lines (Cs I and K I) are weaker in Y dwarfs than in T dwarfs; the far-red optical spectrum of WISE J173835.53+273258.9 is similar to that of late-T dwarfs, but with stronger methane and water features; and we noted the appearance of new absorption features that we propose could be due to hydrogen sulphide. Last but not least, in 2014, WISE J173835.53+273258.9 presented a bluer i - z colour than in 2021 by a factor of 2.8 (significance of 2.5sigma). Thanks to our deep optical images, we found that the 2014 i-band spectrum was contaminated by a galaxy bluer than the Y dwarf.
Conclusions: The optical properties of Y dwarfs presented here pose new challenges to the modelling of grain sedimentation in extremely cool objects. The weakening of the very broad K I resonance doublet due to condensation in dust grains is more abrupt than theoretically anticipated. Consequently, the observed blueing of the z - J and i - z colours of Y dwarfs with respect to T dwarfs is more pronounced than predicted by models and could boost the potential of upcoming deep large-area optical surveys regarding their ability to detect extremely cool objects.

Based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma (programmes GTC27-13B and GTC49-14B - PI: N. Lodieu - GTCMULTIPLE2B-21A and GTC133-21A - PI: E. L. Martín).