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