  Schneider et al. (2020) presented the discovery of WISEA J041451.67-585456.7
and WISEA J181006.18-101000.5, which appear to be the first examples of extreme
T-type subdwarfs (esdTs; metallicity <= -1 dex, T_eff <= 1400 K). Here we
present new discoveries and follow-up of three T-type subdwarf candidates, with
an eye toward expanding the sample of such objects with very low metallicity
and extraordinarily high kinematics, properties that suggest membership in the
Galactic halo. Keck/NIRES near-infrared spectroscopy of WISEA
J155349.96+693355.2, a fast-moving object discovered by the Backyard Worlds:
Planet 9 citizen science project, confirms that it is a mid-T subdwarf. With
H_W2 = 22.3 mag, WISEA J155349.96+693355.2 has the largest W2 reduced proper
motion among all spectroscopically confirmed L and T subdwarfs, suggesting that
it may be kinematically extreme. Nevertheless, our modeling of the WISEA
J155349.96+693355.2 near-infrared spectrum indicates that its metallicity is
only mildly subsolar. In analyzing the J155349.96+693355.2 spectrum, we present
a new grid of low-temperature, low-metallicity model atmosphere spectra. We
also present the discoveries of two new esdT candidates, CWISE
J073844.52-664334.6 and CWISE J221706.28-145437.6, based on their large motions
and colors similar to those of the two known esdT objects. Finding more esdT
examples is a critical step toward mapping out the spectral sequence and
observational properties of this newly identified population.
