We present the results of optical wavelength observations of the unusual SMC
eclipsing binary system HD 5980 obtained in 1999 and 2004--2005. Radial
velocity curves for the erupting LBV/WR object (star A) and its close WR-like
companion (star B) are obtained by deblending the variable emission-line
profiles of N IV and N V lines under the simplistic assumption that these lines
originate primarily in the winds of star A and star B. The derived masses
M_A=58--79 Mo and M_B=51--67 Mo, are more consistent with the stars' location
near the top of the HRD than previous estimates. The presence of a wind-wind
interaction region is inferred from the orbital phase-dependent behavior of He
I P Cygni absorption components. The emission-line intensities continued with
the declining trend previously seen in UV spectra. The behavior of the
photospheric absorption lines is consistent with the results of Schweickhardt
(2002) who concludes that the third object in the combined spectrum, star C, is
also a binary system with P(starC)~96.5 days, e=0.83. The data used in this
paper will be made publicly available for further analysis.Comment: 48 pages, 26 figure
