  We observed ASASSN-19ax during the long outburst in 2021 September-October.
The object has been confirmed to be an SU UMa-type dwarf nova with a superhump
period of 0.1000-0.1001 d. This object showed two post-superoutburst
rebrightenings both in the 2019 and 2021 superoutbursts. These observations
have established that ASASSN-19ax belongs to a group of long-period SU UMa-type
dwarf novae which show multiple rebrightenings. This phenomenon probably arises
from premature quenching of the superoutburst due to the weak 3:1 resonance
near the stability border of the resonance, resulting in a considerable amount
of disk mass after the superoutburst. We noted that ASASSN-19ax is very similar
to QZ Ser, an SU UMa-type dwarf nova with an orbital period of 0.08316 d and an
anomalously hot, bright secondary star, in that both objects showed multiple
post-superoutburst rebrightenings at least once and that they are bright in
quiescence. We expect that the core of the secondary in ASASSN-19ax may be
evolved as in QZ Ser.
