  We have studied the SC (short cadence) Kepler light curve of an SU UMa star,
V1504 Cyg, which extends for a period of about 630 d. All superoutbursts in
V1504 Cyg have turned out to be of the precursor-main type and the superhump
first appears near the maximum of the precursor. The superhumps grow smoothly
from the precursor to the main superoutburst showing that the superoutburst is
initiated by the tidal instability (as evidenced by growing superhump) as
envisioned in the thermal-tidal instability (TTI) model proposed by Osaki
(1989). We have performed power spectral analysis of the light curve of V1504
Cyg. One of outstanding features is an appearance of a negative superhump
extending for around 300 d, well over a supercycle. We have found that an
appearance of the negative superhump tends to reduce the frequency of
occurrence of normal outbursts. Two types of supercycles are recognized in
V1504 Cyg, which are similar to those of the Type L and S supercycles in the
light curve of VW Hyi, a prototype SU UMa star, introduced by Smak (1985). It
is found that the Type L supercycle is the one accompanied with the negative
superhump and the Type S is that without the negative superhump. If we adopt a
tilted disk as an origin of the negative superhump, two types of the
supercycles are understood to be due to a difference in outburst intervals,
which is in turn caused by a difference in mass supply from the secondary to
different parts of the disk. The frequency of the negative superhump varies
systematically during a supercycle in V1504 Cyg. This variation can be used as
an indicator of the disk radius variation and we have found that observed disk
radius variation in V1504 Cyg fits very well with a prediction of the TTI
model.
