AbstractIUE observations of hot companions of Cepheids have been used to determine the temperatures of the companions. For companions on the ZAMS, the temperatures can be used to determine the luminosity of the Cepheids from the inferred absolute magnitude of the companion. The accuracy of this method is comparable to the accuracy of calibration using Cepheids in clusters. An over-tone pulsator (SU Cas) has been identified by this technique. The luminosity of the double mode pulsator Y Car

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... s with that from the PLC, confirming that is a normal Population I Cepheid. The variation of the width of the instability strip (as a function of luminosity) in the HR diagram is confirmed, and overtone and double mode pulsators are shown to be near the blue edge of the instability strip. Comparison between variables and nonvariables in the HR diagram (from IUE and cluster studies) shows very little overlap. The shape of the observed instability region may be determined both by the boundary between the variables and nonvariables and also the location of the tips of the blue loops of evolutionary tracks. Finally, eight Cepheids with hot companions which are evolved beyond the ZAMS have been studied. Half the systems are matched well by evolutionary tracks with little or no core convective overshoot near the main sequence. At least two and possibly four systems, however, cannot be matched by current isochrones.