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Superhumps in the Rarely Outbursting SU UMa-Type Dwarf Nova, HO Delphini

We observed the 1994, 1996 and 2001 outbursts of HO Del. From the detection of secure superhumps, HO Del is confirmed to be an SU UMa-type dwarf nova with a superhump period of 0.06453(6) d. Based on the recent observations and the past records, the outbursts of HO Del are found to be relatively rare, with the shortest intervals of superoutbursts being ~740 d. Among SU UMa-type dwarf novae with similar outburst intervals, the outburst amplitude (~5.0 mag) is unusually small. HO Del showed a rather rapid decay of the superhump amplitudes, and no regrowth of the amplitudes during the later stage, in contrast to the commonly observed behavior in SU UMa-type dwarf novae with long outburst intervals. We positively identified HO Del with a ROSAT X-ray source, and obtained a relatively large X-ray luminosity of 10^(31.1+/-0.2) erg/s. We also performed a literature survey of SU UMa-type dwarf novae, and presented a new set of basic statistics. The SU UMa-type dwarf novae with a brightening trend or with a regrowth of superhumps near the termination of a superoutburst are found to be rather tightly confined in a small region on the (superhump period-supercycle length) plane. These features may provide a better observational distinction for the previously claimed subgroup of dwarf novae (Tremendous Outburst Amplitude Dwarf Novae).