  We have studied the long-term (~ 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
~1 mag on timescales of ~450 days, and spends typically ~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 ~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 Hachisu & Kato (2003a), which provides further support for these
systems containing massive (~1.3 Msun) 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.
