  The secular variation in the interval of outbursts in the following six Z
Cam-type dwarf novae (including the subtype IW And-type) is investigated: Z
Cam, RX And, AH Her, HL CMa, SY Cnc, and WW Cet. An analysis using the $O-C$
diagram shows that the interval of outbursts is not steady in one system. The
outburst properties before standstill are the decrease in outburst interval,
enhancement of the magnitude in quiescence, and disappearance of the long
outburst. Meanwhile, several objects have at least two typical intervals of
outbursts. These characteristics are difficult to be explained only by the
variation in mass transfer from the secondary.

Solar and Stellar Astrophysics