Large amplitude change in spot-induced rotational modulation of the Kepler Ap star KIC 2569073

An investigation of the 200 $\times$ 200 pixel 'superstamp' images of the centres of the open clustersNGC 6791 and NGC 6819 allows for the identification and study of many variable starsthat were not included in the Keplertarget list. KIC 2569073 (V=14.22), is a particularly interesting variable Ap star that we discovered in the NGC 6791 superstamp. With a rotational periodof 14.67 days and 0.034-mag variability, it has one of the largest peak-to-peak variations of any known Ap star. Colour photometry reveals an anti- phase correlationbetween the $ B$ band, and the $V$, $R$ and $ I$ bands. This Ap star is a rotational variable, also known as an $\alpha^2$ CVn, star, and is one of only a handful of Ap stars observed by Kepler. While no change in spot period or amplitude is observed within the 4-year Keplertimeseries, the amplitude shows a large increase compared to ground-based photometry obtained two decades ago.