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Context. Metal-poor brown dwarfs are poorly understood because they are extremely faint and rare. Only a few candidates have been identified as T-type subdwarfs in infrared surveys and their optical properties remain unconstrained.
Aims. We aim to improve the knowledge of the optical properties of T subdwarf candidates to break the degeneracy between metallicity and temperature and to investigate their atmospheric properties.
Methods. Deep z-band images of ten known T subdwarf candidates were collected with the 10.4-m Gran Telescopio Canarias. Low-resolution optical spectra for two of them were obtained with the same telescope. Photometric measurements of the z-band flux were performed for all the targets and they were combined with infrared photometry in J, H, K, W1, and W2 bands from the literature to obtain the colours. The spectra were compared with solar-metallicity T dwarf templates and with laboratory spectra.
Results. We found that the targets segregate into three distinct groups in the W1 - W2 versus z - W1 colour-colour diagram. Group I objects are mixed with solar-metallicity T dwarfs. Group III objects have W1 - W2 colours similar to T dwarfs but very red z - W1 colours. Group II objects lie between Group I and III. The two targets for which we obtained spectra are located in Group I and their spectroscopic properties resemble normal T dwarfs but with water features that are deeper and have a shape akin to pure water.
Conclusions. We conclude that the W1 - W2 versus z - W1 colour-colour diagram is excellent to break the metallicity-temperature degeneracy for objects cooler than L-type ones. A revision of the spectral classification of a T subdwarf might be needed in the future, according to the photometric and spectroscopic properties of WISE1810 and WISE0414 in Group III discussed in this work.