Are the most massive stars in the Local Group truly single?

The chemical evolution and integrated appearance of galaxies are governed by the most massive stars they host. With masses in excess of 50 solar masses (Ms), these stars dominate entire clusters in terms of their energy production. They are ideal progenitors of superluminous / pair-instability supernovae, gamma-ray bursts, and the massive black holes identified through recent gravitational-wave detections. While fundamental to many of our models and predictions, we do not know how massive a star can become. Masses derived from binary orbits generally do not exceed 130Ms. In contrast, a few supposedly-single stars residing in R136 in the Large Magellanic Cloud (LMC) were reported to have masses exceeding 300Ms, twice the canonical upper-mass limit. These mass estimates, however, rely on one risky assumption: that the stars are single. The incidence of multiplicity among the massive stars and the very high crowding in R136 shed doubts on the single-star assumption and warrant a careful investigation of these stars. Several known binaries with components more massive than 100Ms have periods of the order of several months. And yet, the most massive stars in R136 have only been investigated on a time-base of a few weeks and with low-resolution spectroscopy. Here, we propose to collect 3 epochs of HST/STIS spectroscopy of the four most massive candidate stars in the Local Group - R136 a1, a2, a3, and c. With merely 6 orbits, and relying on data quality that is exclusive to the HST/STIS and vital for the study, our analysis will yield statistically-firm constraints on the multiplicity of these objects within a 90% detection limit up to a period of about three years.