We report the discovery of an esdL3 subdwarf , ULAS J020858.62+020657.0 , and a usdL4.5 subdwarf , ULAS J230711.01+014447.1 .
They were identified as L subdwarfs by optical spectra obtained with the Gran Telescopio Canarias , and followed up by optical-to-near-infrared spectroscopy with the Very Large Telescope .
We also obtained an optical-to-near-infrared spectrum of a previously known L subdwarf , ULAS J135058.85+081506.8 , and reclassified it as a usdL3 subdwarf .
These three objects all have typical halo kinematics .
They have T _ { eff } around 2050–2250 K , - 1.8 \leq [ Fe/H ] \leq - 1.5 , and mass around 0.0822–0.0833 M _ { \odot } , according to model spectral fitting and evolutionary models .
These sources are likely halo transitional brown dwarfs with unsteady hydrogen fusion , as their masses are just below the hydrogen-burning minimum mass , which is \sim 0.0845 M _ { \odot } at [ Fe/H ] = - 1.6 and \sim 0.0855 M _ { \odot } at [ Fe/H ] = - 1.8 .
Including these , there are now nine objects in the ‘ halo brown dwarf transition zone ’ , which is a ‘ substellar subdwarf gap ’ that spans a wide temperature range within a narrow mass range of the substellar population .