  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 10 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 $W1 - W2$ vs. $z - W1$ 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 $W1 - W2$ vs. $z - W1$ colour-colour
diagram is excellent to break the metallicity-temperature degeneracy for
objects cooler than L-type. A revision of the spectral classification of 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.
