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 \pm 0.3 kms ^ { -1 } .
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 \sim 1.275 days .
RHy 403A has an absolute magnitude of M _ { I } = 10.85 , consistent with a mass of 0.15 M _ { \odot } .
The systemic mass function has a value { { ( M _ { 2 } sin ( i ) ) ^ { 3 } } \over { ( M _ { 1 } + M _ { 2 } ) ^ { 2 } } } = 0.0085 , which , combined with the non-detection of a secondary peak in the cross-correlation function , implies 0.095 > M _ { 2 } > 0.06 sM _ { \odot } , 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 \sim 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 \sim 2.5 \% for low-mass Hyades stars .
Nine stars have rotational velocities , v \sin ( i ) , exceeding 20 kms ^ { -1 } 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 .