2000MNRAS.316..827R

We have used the HIRES echelle spectrograph on 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 ±0.3km.s^–1. 27 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∼1.275d. RHy 403A has an absolute magnitude of M_I=10.85, consistent with a mass of 0.15M_☉. The systemic mass function has a value M₂sin(i)]³/(M₁+M₂)²=0.0085, which, combined with the non-detection of a secondary peak in the cross-correlation function, implies 0.095>M₂>0.07M_☉, 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 ∼0.25mas. Five other low-mass Hyads are identified as possible spectroscopic binaries, based either on repeat observations or on a 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 30per cent. 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 ∼2.5per cent for low-mass Hyades stars. Nine stars have rotational velocities, vsin(i), exceeding 20km.s^–1, and most of the sample have detectable rotation. We examine the Hα emission characteristics of low-mass cluster members, and show that there is no evidence for a correlation with rotation.