Guinan, DeWarf, Maloney and P. A. Maurone (Physics Dept.), with D. H. Bradstreet (Eastern Coll.) and Y. W. Kang (Sejong U., Seoul, Kor.), are actively researching eclipsing binaries in the Magellanic Clouds. These systems are important laboratories for studying stellar structure, evolution, and mass loss for stars with reduced metallicity. They have obtained IUE SWP (1150-2000Å) spectra of a dozen hot O/B eclipsing systems in the LMC and SMC that have well determined light and radial velocity curves. During Fall 1996 HST FOS and GHRS observations are scheduled for ten of these systems. The chief purpose of the UV spectrophotometry is to determine temperatures and reddenings for these stars. Since these stars radiate most of their energy in the UV, the character of the UV continuum and the presence of highly ionized elements are sensitive measures of stellar temperatures. The UV data have been combined with the UBV (or uvby) photometry and fit with the most recent version (ATLAS13) of Kurucz model atmospheres at metal abundances appropriate for the LMC or SMC. Ground-based light and radial velocity curves are obtained in collaboration with W. Tobin and J. Pritchard (U. Canterbury, Christchurch, NZ), V. Niemela (La Plata, ARG), A. Giménez (INTA/LAEFF, SP), and J.-V. Claussen (Denmark) and provide masses and radii. These data, when combined with the UV spectrophotometry give accurate stellar luminosities. This leads to a first direct Mass-Luminosity relationship, using directly measured masses and luminosities, for stars outside the Milky Way, and extends the parameter space in stellar interior models to chemical compositions different from the Milky Way. These eclipsing binaries can also serve as first class standard candles. Because the uncertainty in the ground-based determination of their temperatures presently contributes the greatest uncertainty in their computed distances, the more precise temperatures obtained from UV spectra, combined with the well determined stellar radii, will allow us to compute the distance moduli to these stars to within mag. This research is supported by NASA grants NAG5-2160 and NSF grant AST-9315365.