We have studied the long-term ( \sim years ) temporal variability of the prototype supersoft X-ray source ( SSS ) CAL 83 in the LMC , using data from the MACHO and OGLE projects .
The CAL 83 light curve exhibits dramatic brightness changes of \sim 1 mag on timescales of \sim 450 days , and spends typically \sim 200 days in the optical low state .
Combined with archival XMM-Newton X-ray observations these represent the most extensive X-ray/optical study to date of this system , and reveal in much greater detail that the X-ray light curve is anti-correlated with the optical behaviour .
This is remarkably similar to the behaviour of the “ transient ” SSS , RX J0513.9-6951 , where the SSS outbursts recur on a timescale of \sim 168 days , and also anti-correlate with the optical flux .
We performed simple blackbody fits to both high and low state X-ray spectra , and find that the blackbody temperature and luminosity decrease when the optical counterpart brightens .
We interpret these long-term variations in terms of the limit-cycle model of ( 15 ) , which provides further support for these systems containing massive ( \sim~ { } 1.3 ~ { } \mathrm { M _ { \sun } } ) white dwarfs .
In addition , we have refined their orbital periods in the MACHO and OGLE-III light curves to values of 1.047529 ( 1 ) days and 0.762956 ( 5 ) days for CAL 83 and RX J0513.9-6951 , respectively .