Optical variability of the LMC supersoft source RX J0513.9-6951

We present optical spectroscopy and photometry of the LMC supersoft source (SSS) RX J0513.9-6951. Through the exceptional monitoring capabilities of the MACHO project, we show the optical history of this object for a≈3 year period. Recurring low states, in which the optical brightness drops by up to a magnitude, are observed at quasi-regular intervals. Analysis of the high state data reveals a small modulation with a semi-amplitude of ≈0.02 magnitudes at P=0d.76278±0d.00005. By considering all the available data, including optical photometry, spectroscopy and X-ray observations, we suggest a theoretical model to explain the fundamental variations exhibited by this source.