Can Very Massive Stars form at the low-metallicity threshold of the nearby Universe?

Very massive stars (VMS, >100 Msun) dominate the ionization and mechanical feedback in star-forming regions for the first few Myr. Evidence for VMS has been found from UV observations of young (< 3 Myr), massive star clusters (>5x10^4 Msun), in the LMC and two nearby starbursts. Larger samples of these stars are needed in order to determine the upper mass limit of the IMF and complete our understanding of massive star evolution. In addition, since JWST will obtain numerous UV rest-frame spectra of young high redshift galaxies, it is essential that we investigate whether VMS are common in local, low metallicity analogs, while we still have access to the UV domain. SBS 0335-052E is one of the nearest most metal-poor analogs and contains several super star clusters (SSC) of 10^5 M_sun. From ground-based optical spectroscopy, candidate Wolf-Rayet (WR) have been found in cluster 3 of the galaxy (SSC3). Given its young age, large mass, and WR-like signatures, SSC3 could host VMS. We request co-spatial STIS G140L UV + G430L optical 52x0.2 long-slit spectroscopy of SSC3 in SBS 0335-052E. The requested observations will cover the high-ionization UV emission lines of C IV, He II, and O III], and the optical He II 4686 line. Dust attenuation will be obtained from the UV to optical He II ratio. We will use the co-spatial UV + optical data to test if WR stars alone can explain the observed properties of SSC3 or if VMS are necessary. The observations will provide important constraints to state-of-the-art spectral synthesis models of the kind that will be used to interpret rest-frame UV spectra of the most distant galaxies, with future large telescopes.