On the nature of long-period dwarf novae with rare and low-amplitude outbursts

This chapter report on the optical photometry, spectroscopy of the three long-period dwarf novae 1SWASP J1621, BD Pav, and V364 Lib. They were recognized as weird dwarf novae because of their infrequent and low-amplitude outbursts and highly ionized emission lines in the outburst state. By the data analyses and the modeling of the phase-averaged orbital profile, it was revealed that 1SWASP J1621 and BD Pav very high inclinations and possibly low mass-transfer rates and that V364 Lib has an F-type (bright, hot, and large) companion star, a massive white dwarf, and a very long orbital period of around 17 h. We consider that the infrequent outbursts come from the low mass transfer rate and/or the large accretion disk, and that the small amplitude of outbursts originates from the high inclination and/or the bright companion star. Also, the highly-ionized emission lines observed during outburst would be attributed to the disk wind and/or the high-temperature region in the vicinity of the massive white dwarf. We conclude that the outbursts of the three systems can be explained within the framework of thermal-viscous instability as those in normal dwarf novae.