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The Lyman continuum (LyC = hydrogen-ionizing radiation at wavelengths < 912 Ang) of galaxies plays a fundamental role in determining the physical and observational properties of the ISM and IGM comma including cosmic reionization. Yet it is basically inaccessible to direct observations comma and one therefore heavily relies on theoretical comma yet untested predictions from synthesis models. Furthermore the ionizing spectra differ significantly between different models comma with important implications on widely used emission line diagnostics and other observables. We here propose to observe for the first time the shape of the ionizing continuum of several star-forming galaxies shortward of (from virgul600 to 912 Angstroem) and across the Lyman break. This can be achieved with COS by targeting LyC emitters at zvirgul0.6-0.8. The observations will measure for the first time stellar and nebular emission features in the LyC comma and thus provide the first direct constraints for stellar population models in this spectral range. The proposed observations will provide unique insight on the hardness of the ionizing spectra of star-forming galaxies comma with numerous possible implications on our understanding of the emission line properties of distant galaxies comma sources of cosmic reionization comma emission line diagnostics comma and related topics.