A New Orbital Ephemeris and Reinterpretation of Spectroscopic Data for the Supersoft X-ray Binary RX J0513.9-6951

We have analyzed nearly eight years of MACHO optical photometry of the supersoft X-ray binary RX J0513.9-6951 and derived a revised orbital period and ephemeris. Previously published velocities are reinterpreted using the new ephemeris. We show that the spectroscopic characteristics of the system depend strongly on whether the system is in a high or low optical state. We also discuss the properties of the source's high/low optical states and its long-term light curve. Evidence for a 83.3-day periodicity in the photometry is presented.