We carried out the photometric observations of the SU UMa-type dwarf nova ER UMa during 2011 and 2012, which showed the existence of persistent negative superhumps even during the superoutburst. We performed two-dimensional period analysis of its light curves by using a method called "least absolute shrinkage and selection operator" (Lasso) and "phase dispersion minimization" (PDM) analysis, and we found that the period of negative superhumps systematically changed between a superoutburst and the next superoutburst. The trend of the period change can beinterpreted as reflecting the change of the disk radius. This change of the disk radius is in good agreement with the predicted change of the disk radius by the thermal-tidal instability (TTI) model. The normal outbursts within a supercycle showed a general trend that the rising rate to maximum becomes slower as the next superoutburst approaches. The change can be interpreted as the consequence of the increased gas-stream flow onto the inner region of the disk as the result of the tilted disk. Some of the superoutbursts were found to be triggered by a precursor normal outburst when the positive superhumps appeared to develop. The positive and negative superhumps co-existed during the superoutburst. The positive superhumps were prominent only during four or five days after the supermaximum, while the signal of the negative superhumps became strong after the middle phase of the superoutburst plateau. A simple combination of the positive and negative superhumps was found to be insufficient in reproducing the complex profile variation. We were able to detect the developing phase of positive superhumps (stage A superhumps) for the first time in ER UMa-type dwarf novae. Using the period of stage A superhumps, we obtained a mass ratio of 0.100(15), which indicates that ER UMa is on the ordinary evolutional track of CVs.
Authors: Tomohito Ohshima, Taichi Kato, Elena Pavlenko, Hidehiko Akazawa, Kazuyoshi Imamura, Kenji Tanabe, Enrique de Miguel, William Stein, Hiroshi Itoh, Franz-Josef Hambsch, Pavol A. Dubovsky, Igor Kudzej, Thomas Krajci, Alex Baklanov, Denis Samsonov, Oksana Antonyuk, Viktor Malanushenko, Maksim Andreev, Ryo Noguchi, Kazuyuki Ogura, Takashi Nomoto, Rikako Ono, Shin'ichi Nakagawa, Keisuke Taniuchi, Tomoya Aoki, Miho Kawabata, Hitoshi Kimura, Kazunari Masumoto, Hiroshi Kobayashi, Katsura Matsumoto, Kazuhiko Shiokawa, Sergey Yu. Shugarov, Natalia Katysheva, Irina Voloshina, Polina Zemko, Kiyoshi Kasai, Javier Ruiz, Hiroyuki Maehara, Natalia Virnina, Jani Virtanen, Ian Miller, Boyd Boitnott, Colin Littlefield, Nick James, Tamas Tordai, Fidrich Robaert, Stefono Padovan, Atsushi