| Abstract
| - The colliding-wind binary system WR 140 (HD 193793, WC7pd+O4-5, P = 7.94 yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35 short- wavelength high-resolution spectra. An absorption-line radial-velocity solution is obtained from the photospheric lines of the O component, by comparison with a single O star. The resulting orbital parameters, e = 0.87 ± 0.05, ω = 31° ± 9° and $K_{\rm O star}$ = 25 ± 15 km s -1, confirm the large eccentricity of the orbit, within the uncertainties of previous optical studies. This brings the weighted mean UV-optical eccentricity to e = 0.85 ± 0.04. Occultation of the O-star light by the WC wind and the WC+O colliding-wind region results into orbital modulation of the P-Cygni profiles of the C ii, C iv and Si iv resonance lines. Near periastron passage, the absorption troughs of those resonance-line profiles increase abruptly in strength and width, followed by a gradual decrease. In particular, near periastron the blue black-edges of the P-Cygni absorption troughs shift to larger outflow velocities. We discuss that the apparently larger wind velocity and velocity dispersion observed at periastron could be explained by four phenomena: ( i) geometrical resonance-line eclipse effects being the main cause of the observed UV spectral variability, enhanced by sightline crossing of the turbulent wind-wind collision zone; ( ii) the possibility of an orbital-plane enhanced WC7 stellar wind; ( iii) possible common-envelope acceleration by the combined WC and O stellar radiation fields; and ( iv) possible enhanced radiatively driven mass loss due to tidal stresses, focused along the orbiting line of centers.
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