We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of the superhump period is found to be composed of three distinct stages: an early evolutionary stage with a longer superhump period, a middle stage with systematically varying periods, and a final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods of less than 0.08 d show positive period derivatives. We present observational characteristics of these stages and give greatly improved statistics. Contrary to an earlier claim, we found no clear evidence for a variation of period derivatives among different superoutbursts of the same object. We present an interpretation that the lengthening of the superhump period is a result of the outward propagation of an eccentricity wave, which is limited by the radius near the tidal truncation. We interpret that late-stage superhumps are rejuvenated excitation of a 3:1 resonance when superhumps in the outer disk are effectively quenched. The general behavior of the period variation, particularly in systems with short orbital periods, appears to follow a scenario proposed in Kato, Maehara, and Monard (2008, PASJ, 60, L23). We also present an observational summary of WZ Sge-type dwarf novae. Many of them have shown long-enduring superhumps during a post-superoutburst stage having longer periods than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently with the mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives, and are excellent candidates for those systems around or after the period minimum of evolution of cataclysmic variables.

No abstract

Context. AG Peg is known as the slowest symbiotic nova, which experienced its nova-like outburst around 1850. After 165 years, during June of 2015, it erupted again showing characteristics of the Z And-type outburst. Aims. The primary objective is to determine basic characteristics, the nature and type of the 2015 outburst of AG Peg. Methods. We achieved this aim by modelling the spectral energy distribution using low-resolution spectroscopy (330-750 nm; R = 500-1000), medium-resolution spectroscopy (420-720 nm; R ∼ 11000), and U BVR C I C photometry covering the 2015 outburst with a high cadence. Optical observations were complemented with the archival HST and FUSE spectra from the preceding quiescence. Results. During the outburst, the luminosity of the hot component was in the range of 2-11×10 37 (d/0.8 kpc) 2 erg s −1 , being in correlation with the light curve (LC) profile. To generate the maximum luminosity by the hydrogen burning, the white dwarf (WD) had to accrete at ∼ 3 × 10 −7 M ⊙ yr −1 , which exceeds the stable-burning limit and thus led to blowing optically thick wind from the WD. We determined its mass-loss rate to a few ×10 −6 M ⊙ yr −1 . At the high temperature of the ionising source, 1.5 − 2.3 × 10 5 K, the wind converted a fraction of the WD's photospheric radiation into the nebular emission that dominated the optical. A one order of magnitude increase of the emission measure, from a few ×10 59 (d/0.8 kpc) 2 cm −3 during quiescence, to a few ×10 60 (d/0.8 kpc) 2 cm −3 during the outburst, caused a 2 mag brightening in the LC, which is classified as the Z And-type of the outburst. Conclusions. The very high nebular emission and the presence of a disk-like H i region encompassing the WD, as indicated by a significant broadening and high flux of the Raman-scattered O vi 6825 Å line during the outburst, is consistent with the ionisation structure of hot components in symbiotic stars during active phases.