  We have used the HIRES echelle spectrograph in the Keck I telescope to obtain
high-resolution spectroscopy of 51 late-type M dwarfs in the Hyades cluster.
Cross-correlating the calibrated data against spectra of white dwarfs allows us
to determine heliocentric velocities with an accuracy of +/- 0.3 km/s. Twenty
seven stars were observed at two epochs in 1997; two stars, RHy 42 and RHy 403,
are confirmed spectroscopic binaries. RHY 42 is a double-lined, equal-mass
system; RHy 403 is a single-lined, short-period binary, P ~ 1.275 days. RHy
403A has an absolute magnitude of M_I = 10.85, consistent with a mass of 0.15
M(Sun). The systemic mass function has a value of 0.0085, which, combined with
the non-detection of a secondary peak in the cross-correlation function,
implies 0.095 > M_2 > 0.06 M (Sun), and the strong possibility that the
companion is the first Hyades brown dwarf to be identified. Unfortunately, the
maximum expected angular separation in the system is only 0.25 mas. Five other
low-mass Hyads are identified as possible spectroscopic binaries, based either
on repeat observations or comparison between the observed radial velocity and
the value expected for Hyades cluster members. Combined with HST imaging data,
we infer a binary fraction between 23 and 30 . All of the stars are
chromospherically active. RHy 281 was caught in mid-flare and, based on that
detection, we estimate a flaring frequency of 2.5% for low-mass Hyades stars.
Nine stars have rotational velocities, v sin(i), exceeding 20 km/s and most of
the sample have detectable rotation. We examine the H-alpha emission
characteristics of low-mass cluster members and show that there is no evidence
for a correlation with rotation.
