  We present spectroscopic observations of the black-hole binary V4641
Sagittarii, obtained between 4th July 2004 and 28th March 2005, which cover the
minor outburst of the star in early July 2004 and quiescence variations on 19
nights scattered over six months. During the outburst, the star peaked
approximately 3 magnitudes brighter than usual, and our spectra were dominated
by broad hydrogen, helium and iron emission lines. The very first spectra
showed P Cygni profiles, which disappeared within a few hours, indicating rapid
changes in matter ejection. The H-alpha line had multiple components, one being
a broad blue-shifted wing exceeding 5000 km/s. During a simultaneously observed
10-min photometric flare-up, the equivalent width of the H-alpha line
temporarily decreased, implying that it was a flare of the continuum. The
overall spectral appearance was similar to that observed in the 1999 September
active phase, which suggests that similar mass-ejection processes were
associated with both eruptions. In quiescence, the spectra were those of the
early-type secondary star showing its orbital motion around the primary. By
measuring cross-correlation radial velocities, we give an improved set of
spectroscopic elements. Whereas we measure the same velocity amplitude
(K_2=211.3+/-1.0 km/s), within errors, as Orosz et al. (2001), our
centre-of-mass velocity (v_gamma=72.7+/-3.3 km/s) differs significantly from
the previously published value (107.4+/-m2.9 km/s). However, we find evidence
that the difference is caused by a systematic error in data reduction in the
previous study, rather than by gravitational effects of an invisible third
component.
