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

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.