Multiple timescales in cataclysmic binaries. The low-field magnetic dwarf nova DO Draconis

Aims: We study the variability of the cataclysmic variable DO Dra, on time-scales of between minutes and decades.
Methods: The observations were obtained at the Korean 1 m telescope at the Mt. Lemmon in 2006-2007, 14 observational runs cover 45 h. The table of individual observations is available electronically. Additionally, we have used 1509 patrol observations from the international AFOEV and VSOLJ databases.
Results: The characteristic decay time dt/dm = 0.902(3) days/mag was estimated from our 3 nights of CCD R observations, which cover the descending branch of the outburst in 2006. The range of the outburst cycle is from 311^d to 422^d, contrary to a previous estimate of 870^d. The “quiescent” data show a photometric wave with a cycle ~ 303(15)^d. We analyzed the profile of the “composite” (or “mean”) outburst. We discovered however, that a variety of different outburst heights and durations had occurred, contrary to theoretical predictions. The analysis of the historical data has shown a correlation between the decay time dt/dm and the outburst maximum brightness with a slope d(dt/dm)/dm = 0.37(9). With increasing maximum brightness, we find that the decay time also increases; this is in contrast to the model predictions, which indicate that outbursts should have a constant shape. This is interpreted as representing the presence of outburst-to-outburst variability of the magnetospheric radius. A presence of a number of missed weak narrow outbursts is predicted from this statistical relationship. We tabulate characteristics of the “quasi-orbital” variations, which indicate that an amplitude maximum occurs between quiescence and the outburst peak. The semi-amplitude of the spin variability does not exceeded 0.02 mag. A new type of variability is detected, during 3 subsequent nights in 2007: periodic (during one nightly run) oscillations with rapidly-decreasing frequency from 86 to 47 cycles/day and a semi-amplitude increasing from 0.06m to 0.10m, during a monotonic brightness increase from 14.27m to 14.13m. This phenomenon was observed only during an unusually prolonged event of ~1 mag brightening in 2007 (lasting till autumn), during which no (expected) outburst was detected. We refer to this behaviour as to the “transient periodic oscillations” (TPO). We attribute the frequency decrease to “beat”-type of the variability, probably caused by irradiation of a cloud that is spiralling down to the white dwarf. Its frequency would then increase and coverge towards the spin frequency. To study this new and interesting phenomenon, new regular photometric and spectral (in a “target of opportunity” mode) observations are required.

Table 3 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/486/855