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Cepheids play an important part in the Hubble Constant tension comma and in evolutionary calculation benchmarks comma which include comparisons with LMC Cepheids. In some cases comma they lead to exotic end-stage objects. An important HST UV legacy is the measurement of the Cepheid masses comma the fundamental parameter in stellar evolution. The HST high resolution E140H spectra requested in this proposal (a unique capability of HST) will measure the orbital velocity amplitude of hot companions of three Cepheids comma V1334 Cyg comma S Mus comma and SU Cyg. This can be combined with recent infrared interferometry with the Very Large Telescope Interferometer (VLTI) and CHARA which has resolved the systems comma providing visual orbits to augment the spectroscopic orbits. From this combination both masses and distances for the Cepheids have been derived comma which already challenge current evolutionary tracks comma even those incorporating rotation and core convective overshoot on the main sequence. The observations detailed in this proposal will improve the orbital solution through more accurate orbital velocity amplitudes of the hot companions. In addition for SU Cyg the phasing of the short period orbit of the companion will be more precise. The requested observations are predicted to yield mass solutions accurate to a few percent. These will be the most accurate Cepheid mass determinations available to quantitatively test the underlying physics at this stage.