  We present high angular resolution, near-infrared imaging and spectroscopy of
a low-mass companion to the lithium-depleted, double-line spectroscopic binary
HBC 425 (St 34) obtained using the Near Infrared Spectrograph (NIRSPEC) and the
Keck II adaptive optics system. Positioned 1.23" southeast of the primary pair,
the companion, HBC 425C, is ~2.4 magnitudes fainter at 2.2 microns.
Moderate-resolution (R~2500) J- and K-band spectroscopy reveal HBC 425C to have
an M5.5 (+/-0.5) spectral type. Comparisons with pre-main sequence evolutionary
models imply a mass of ~0.09 M(Sun) and ages of 8-10 Myr, assuming the nominal
distance of Taurus-Auriga (~140 pc), or ~25 Myr if placed at ~90 pc. We also
present high dispersion, optical spectra of HBC 425 and HBC 425C obtained using
the High Resolution Echelle Spectrometer (HIRES) on Keck I. We detect strong Li
I 6708 absorption in the spectrum of HBC 425C. Using curves of growth for the
Li I 6708 doublet, we estimate its abundance level to lie between log N(Li)=1.9
and 3.1 dex. The spectrum of HBC 425 exhibits Ca II H & K, He I 5876, 6678, and
strong Balmer line emission, consistent with accretion. We place more
restrictive upper limits on the surface abundance of lithium and find that HBC
425 retains less than ~0.1% of its primordial abundance. The presence of
lithium in the photosphere of HBC 425C does not resolve the discrepancy between
isochronal and lithium depletion ages for the primary pair. However, if lithium
were depleted relative to interstellar abundance levels, even minimally,
considerable support would be gained for the more advanced age of this
hierarchical triple system.
