A low-metallicity massive contact binary undergoing slow Case A mass transfer: A detailed spectroscopic and orbital analysis of SSN 7 in NGC 346 in the SMC

Most massive stars are believed to be born in close binary systems where they can exchange mass, which impacts the evolution of both components. Their is great interest search for progenitors gravitational waves. However, there unknowns physics mass transfer as observational examples rare, especially at low metallicity. Nearby low-metallicity environments particularly interesting hunting grounds interacting act closest proxy early universe we resolve individual stars. Using multi-epoch spectroscopic data, complete a consistent spectral and orbital analysis early-type SSN 7 hosting ON If$^\ast$+O5.5 V((f)) star. these detailed results, constrain an evolutionary scenario that help us understand We were able derive reliable radial velocities two components from used parameters. The data covers UV, optical, near-IR, allowing with stellar atmosphere code, PoWR. Given parameters, interpreted results using models. system have comparable luminosities ${\log (L_1/L_{\odot}) = 5.75}$ (L_2/L_{\odot}) 5.78}$ primary secondary, respectively, but different temperatures (${T_1=43.6\,\mathrm{kK}}$ ${T_2=38.7\,\mathrm{kK}}$). ($32\,M_{\odot}$) less than secondary ($55\,M_{\odot}$), suggesting exchange. estimates confirmed by analysis. revisited period $3\,\mathrm{d}$. Our models also predict Currently, contact undergoing slow Case A phase, making it most [Abridged]