ar X iv : a st ro - p h / 05 03 41 7 v 1 1 8 M ar 2 00 5 FUSE Observations of RX J 0513 . 9 − 6951

FUSE observations were obtained in July 2003 during 1.2 cycles of the 0.76-day binary orbit of RX J0513.9−6951. Radial velocity measurements of the broad O VI emission profile show a semiamplitude of K∼26 km s −1 , which is much smaller than the value of 117 km s −1 measured from 2001 FUSE data. Narrow O VI emissions show no measurable velocity variation. The mean velocity of the broad O VI emission is red-shifted by ∼500 km s −1 with respect to both the systemic and narrow emission-line velocities. Spectral difference plots show phase-related changes in the broad emission profile. Other phase-related changes such as line and continuum variations are also smaller than in the 2001 spectra. We describe a moving broad absorption feature near 1020Å as possible O VI outflow associated with a precessing jet. We discuss the implications for the stellar masses if the 2003 broad O VI velocities outline the compact star's orbital motion.