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The first glimpse of the spectral properties of zvirgul7-10 galaxies has emerged in the last several years. Deep UV spectra have revealed intense emission from nebular CIIICORCHETE_CLOSE and CIV comma implying a hard radiation field that is rarely seen at lower redshifts. Unfortunately comma we are currently completely unprepared to interpret these features comma owing largely to shortcomings in our understanding of the low metallicity stellar populations thought to power this line emission. Fortunately comma HST-COS has uncovered a sample of nearby metal poor galaxies with UV nebular line spectra approaching those seen at z>6. The discovery of these galaxies opens the door for a comprehensive investigation of the low metallicity stellar populations that likely dominate at z>6. Here we propose to obtain ultra-deep (6 orbit) COS-G160M spectra capable of measuring stellar photospheric and wind absorption features in a sample of 9 metal poor galaxies with intense CIV and CIIICORCHETE_CLOSE emission. The data will provide quantitative constraints on the metallicity of the massive stars required to power the hard radiation fields comma establishing the best empirical baseline for interpreting reionization-era spectra. The proposed sample size is chosen to reveal how the ionizing spectrum varies with stellar metallicity and age in the parameter range expected at z>6. For each galaxy comma we will simultaneously fit the stellar absorption features and nebular lines comma providing a powerful stress test of low-metallicity models for massive stars. If this UV spectral database is not obtained while COS is still functioning comma the interpretation of reionization era galaxy spectra will be severely jeopardized throughout the JWST era.