  Stellar rotation plays a key role in stellar activity. The rotation period
could be detected through light curve variations caused by starspots. Kepler
provides two types of light curves, one is the Pre-search Data Conditioning
(PDC) light curves, the other is the Simple Aperture Photometer (SAP) light
curves. Compared with the PDC light curves, the SAP light curves keep the
long-term trend, relatively suitable for searches of long period signals.
However, SAP data are inflicted by some artefacts such as quarterly rolls and
instrumental errors, making it difficult to find the physical periods in the
SAP light curves. We explore a systematic approach based on the light curve
pre-processing, period detection and candidate selection. We also develop a
simulated light curve test to estimate our detection limits for the SAP-like
LCs. After applying our method to the raw SAP light curves, we found more than
1000 main-sequence stars with the period longer than 30 days, 165 are newly
discovered. Considering the potential flaw of the SAP, we also inspect the
newly found objects with photometry methods, and most of our periodical signals
are confirmed.
