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With years of Kepler data currently available, it can now be attempted tomeasure variations in planetary transit depths over time. To do so, it is ofprimary importance to understand which systematic effects may affect themeasurement of transits. We aim to measure the stability of Kepler measurementsover years of observations. We present a study of the depth of about 500transit events of the Hot Jupiter HAT-P-7b, using 14 quarters (Q0-Q13) of datafrom the Kepler Satellite. We find a systematic variation in the depth of theprimary transit, related to quarters of data and recurring yearly. Theseseasonal variations are about 1%. Within seasons, we find no evidence fortrends. We speculate that the cause of the seasonal variations could be unknownfield crowding or instrumental artifacts. Our results show that care must betaken when combining transits throughout different quarters of Kepler data.Measuring the relative planetary radius of HAT-P-7b without taking thesesystematic effects into account leads to unrealistically low error estimates.This effect could be present in all Kepler targets. If so, relative radiusmeasurements of all Hot Jupiters to a precision much better than 1% areunrealistic.

Investigation of systematic effects in Kepler data: Seasonal variations in the light curve of HAT-P-7b