Abstract
KU Cyg is an eclipsing binary consisting of a F-type star accreting through a large accretion disk from a K5III red giant. Here we present the discovery of a 5-yr dip around 1900 found from its 100 yr DASCH (Digital Access to a Sky Century at Harvard) light curve. It showed a ~0.5 mag slow fading from 1899 to 1903, and brightened back around 1904 on a relatively shorter timescale. The light curve shape of the 1899-1904 fading-brightening event differs from the dust production and dispersion process observed in R Coronae Borealis (RCB) stars, which usually has a faster fading and slower recovery, and for KU Cyg is probably related to the accretion disk surrounding the F star. The slow fading in KU Cyg is probably caused by increases in dust extinction in the disk, and the subsequent quick brightening may be due to the evaporation of dust transported inwards through the disk. The extinction excess which caused the fading may arise from increased mass transfer rate in the system, or from dust clump ejections from the K giant.
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