* Astronomy

Title: Superoutburst of WZ Sge-type Dwarf Nova Below the Period Minimum: ASASSN-15po
Author: Kosuke Namekata, Keisuke Isogai, Taichi Kato, Colin Littlefield, Katsura Matsumoto, Naoto Kojiguchi, Yuki Sugiura, Yusuke Uto, Daiki Fukushima, Taiki Tatsumi, Eiji Yamada, Taku Kamibetsunawa, Enrique de Miguel, William L. Stein, Richard Sabo, Maksim V. Andreev, Etienne Morelle, E. P. Pavlenko, Julia V. Babina, Alex V. Baklanov, Kirill A. Antonyuk, Okasana I. Antonyuk, Aleksei A. Sosnovskij, Sergey Yu. Shugarov, Polina Yu. Golysheva, Natalia G. Gladilina, Ian Miller, Vitaly V. Neustroev, Vahram Chavushyan, Jose R. Valdes, George Sjoberg, Yutaka Maeda, Hiroshi Itoh, Gianluca Masi, Raul Michel, Pavol A. Dubovsky, Seiichiro Kiyota, Tamas Tordai, Arto Oksanen, Javier Ruiz, Daisaku Nogami

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. In this outburst, we observed early superhumps and growing (stage A) superhumps with periods of 0.050454(2) and 0.051809(13) d, respectively. We estimated that the mass ratio of secondary to primary (q) is 0.0699(8) by using P_orb and a superhump period P_SH of stage A. ASASSN-15po [P_orb~ 72.6 min] is the first DN with the orbital period between 67--76 min. Although the theoretical predicted period minimum P_min of hydrogen-rich cataclysmic variables (CVs) is about 65--70 min, the observational cut-off of the orbital period distribution at 80 min implies that the period minimum is about 82 min, and the value is widely accepted. We suggest the following four possibilities: the object is (1) a theoretical period minimum object (2) a binary with a evolved secondary (3) a binary with a metal-poor (Popullation II) secondary (4) a binary which was born with a brown-dwarf donor below the period minimum.

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