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We report on a superoutburst of a WZ Sge-type dwarf nova (DN), ASASSN-15po.The light curve showed the main superoutburst and multiple rebrightenings. Inthis outburst, we observed early superhumps and growing (stage A) superhumpswith periods of 0.050454(2) and 0.051809(13) d, respectively. We estimated thatthe mass ratio of secondary to primary ($q$) is 0.0699(8) by using $P_{\rmorb}$ and a superhump period $P_{\rm SH}$ of stage A. ASASSN-15po [$P_{\rm orb}\sim$ 72.6 min] is the first DN with the orbital period between 67--76 min.Although the theoretical predicted period minimum $P_{\rm min}$ ofhydrogen-rich cataclysmic variables (CVs) is about 65--70 min, theobservational cut-off of the orbital period distribution at 80 min implies thatthe period minimum is about 82 min, and the value is widely accepted. Wesuggest the following four possibilities: the object is (1) a theoreticalperiod minimum object (2) a binary with a evolved secondary (3) a binary with ametal-poor (Popullation II) seconday (4) a binary which was born with abrown-dwarf donor below the period minimum.

Superoutburst of WZ Sge-type Dwarf Nova Below the Period Minimum: ASASSN-15po