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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 superhumpperiod of 0.1000-0.1001 d. This object showed two post-superoutburstrebrightenings both in the 2019 and 2021 superoutbursts. These observationshave established that ASASSN-19ax belongs to a group of long-period SU UMa-typedwarf novae which show multiple rebrightenings. This phenomenon probably arisesfrom premature quenching of the superoutburst due to the weak 3:1 resonancenear the stability border of the resonance, resulting in a considerable amountof disk mass after the superoutburst. We noted that ASASSN-19ax is very similarto QZ Ser, an SU UMa-type dwarf nova with an orbital period of 0.08316 d and ananomalously hot, bright secondary star, in that both objects showed multiplepost-superoutburst rebrightenings at least once and that they are bright inquiescence. We expect that the core of the secondary in ASASSN-19ax may beevolved as in QZ Ser.

ASASSN-19ax: SU UMa-type dwarf nova with a long superhump period and post-superoutburst rebrightenings