Breelyn Cocke, Philip Massey, Nidia I. Morrell, Laura R. Penny, Kathryn F. Neugent, Jan J. Eldridge, Michal K. Szymanski, Andrzej Udalski, Laurella C. Marin

Wolf-Rayet (WR) stars are helium-burning, evolved massive stars which have had most of their hydrogen-rich outer layers removed either through stellar winds and/or binary stripping. Here we report on LMC173-1, a WN3+O binary located in the Large Magellanic Cloud (LMC). Using spectra obtained from the Magellan and Gemini-S telescopes, we have derived system parameters for this intriguing binary. The WR star's mass is only 43% that of its companion, and we argue that this requires binary evolution rather than mass loss by stellar winds alone, given the metallicity of the LMC. The stars are close enough to each other with their 3.52 day period that the O star is actually orbiting within the wind of the WR star, as is the case for other well-known WR+O systems, such as V444 Cyg. As a result, high precision OGLE photometry reveals a WR atmospheric eclipse, as well as a 7-8 millimag ellipsoidal modulation due primarily to the tidal distortion of the O star. Modeling the light curve allows us to estimate the orbital inclination. Derivation of stellar parameters suggests neither component is filling its Roche-lobe surface today. The O star is spinning much faster than synchronous rotation. Using BPASS v2.2 binary models, we discuss the probable evolutionary history of the system. The WR progenitor likely underwent Case A Roche-lobe overflow (RLOF) before leaving the main-sequence. As it lost its H-rich envelope, it became a WN-type WR. The resulting system is a binary with similar luminosities but very different radii, representing a post-RLOF phase.