Abstract
Following previous IUE-based spectroscopic studies of WR+O binaries in the Galaxy and in the SMC, we present a similar study of the two systems, Br 5 [O3 If^*(+O) + O8-B0 I(+OB?)] and Br 31 [WC4(+O?) + O8 I:] in the Large Magellanic Cloud. We detect wind eclipse effects in the WC4+O (P = 3.033 d) pair in Br 31 similar to, but weaker than those observed in the Small Magellanic Cloud system Sk 188 (WO4+O4 V). A low-amplitude ( ~ 0 km s-1) variation in the radial velocity of UV photospheric absorption lines and the O V 1371 emission with the 3 day period is detected. The radial velocity variations of the photospheric lines may be due to the superposition of the stationary set of absorption lines belonging to the O8 I: star and a broader set of lines belonging to the O-type companion in the close binary pair. The UV continuum energy distribution of Br 31 also supports the optical results that the system contains at least 3 bright stars, one of which is a late O-type supergiant. Contrasting with Br 31, the absence of significant Si IV 1400 Å emission in the UV spectrum of Br 5 contradicts the results from optical spectroscopy that imply that it is triple, with the presence of a late O-type supergiant in the system. Orbital phase-coverage of the IUE observations does not allow the detection of possible atmospheric eclipse effects in Br 5, with P = 2.001 d, but radial velocity variations attributable to orbital motion of the O3 If^* star are detected.
