  Some close binaries of the beta Lyrae type show photometric cycles longer
than the orbital one, which are possibly related to changes in their accretion
disks. We aim to understand the short- and long-scale changes observed in the
light curve of the eclipsing system OGLE-BLG-ECL-157529. In particular, we want
to shed light on the contribution of the disk to these changes, especially
those related to the long cycle, occurring on timescales of hundreds of days.
We studied I-band OGLE photometric times series spanning 18.5 years,
constructing disk models by analyzing the orbital light curve at 52 consecutive
epochs. An optimized simplex algorithm was used to solve the inverse problem by
adjusting the light curve with the best stellar-orbital-disk parameters for the
system. We applied principal components analysis to the parameters to evaluate
their dependence and variability. We constructed a description of the mass
transfer rate in terms of disk parameters. We find that the light variability
can be understood in terms of a variable mass transfer rate and variable
accretion disk. The system brightness at orbital phase 0.25 follows the long
cycle and is correlated with the mass transfer rate and the disk thickness. The
long-cycle brightness variations can be understood in terms of differential
occultation of the hotter star by a disk of variable thickness. Our model fits
the overall light curve during 18.5 years well, including epochs of reversal of
main and secondary eclipse depths. The disk radius cyclically change around the
tidal radius, decoupled from changes in the mass transfer rate or system
brightness, suggesting that viscous delay might explain the non-immediate
response. Although the disk is large and fills a large fraction of the hot star
Roche lobe, Lindblad resonance are far beyond the disk, excluding viscous
dissipation as a major source of photometric variability.

Solar and Stellar Astrophysics