Context. The disk instability model attributes the outbursts of dwarf novae to a thermal-viscous instability of their accretion disk, an instability to which nova-like stars are not subject. Aims. We aim to test the fundamental prediction of the disk instability model: the separation of cataclysmic variables (CVs) into nova-likes and dwarf novae depending on orbital period and mass transfer rate from the companion. Methods. We analyse the lightcurves from a sample of ≈ 130 CVs with a parallax distance in the Gaia DR2 catalogue to derive their average mass transfer rate. The method for converting optical magnitude to mass accretion rate is validated against theoretical lightcurves of dwarf novae. Results. Dwarf novae (resp. nova-likes) are consistently placed in the unstable (resp. stable) region of the orbital period -mass transfer rate plane predicted by the disk instability model. None of the analysed systems present a challenge to the model. These results are robust against the possible sources of error and bias that we investigated. Lightcurves from Kepler or, in the future, the LSST or Plato surveys, could alleviate a major source of uncertainty, the irregular sampling rate of the lightcurves, assuming good constraints can be set on the orbital parameters of the CVs that they happen to target. Conclusions. The disk instability model remains the solid base on which to construct the understanding of accretion processes in cataclysmic variables.

We present a statistical study of all measurable photometric features of a large sample of dwarf novae during their outbursts and superoutbursts. We used all accessible photometric data for all our objects to make the study as complete and up-to-date as possible. Our aim was to check correlations between these photometric features in order to constrain theoretical models which try to explain the nature of dwarf novae outbursts. We managed to confirm a few of the known correlations, that is the Stolz and Schoembs Relation, the Bailey Relation for long outbursts above the period gap, the relations between the cycle and supercycle lengths, amplitudes of normal and superoutbursts, amplitude and duration of superoutbursts, outburst duration and orbital period, outburst duration and mass ratio for short and normal outbursts, as well as the relation between the rise and decline rates of superoutbursts. However, we question the existence of the Kukarkin-Parenago Relation but we found an analogous relation for superoutbursts. We also failed to find one presumed relation between outburst duration and mass ratio for superoutbursts. This study should help to direct theoretical work dedicated to dwarf novae.

Context. The fast temporal evolution of the ejecta morphology of novae can be considered as an important test bench for studying the shaping of many kinds of nebulae. V1280 Sco is one of the slowest dust-forming nova ever historically observed that has experienced a particularly long common-envelope phase. Aims. We performed multi-epoch high-spatial resolution observations of the circumstellar dusty environment of V1280 Sco to investigate the level of asymmetry of the ejecta. Methods. We observed V1280 Sco in 2009, 2010 and 2011 (from t = 877 days after discovery until t = 1664 d) using unprecedented high angular resolution techniques. We used the NACO/VLT adaptive optics system in the J, H and K bands, together with contemporaneous VISIR/VLT mid-IR imaging that resolved the dust envelope of V1280 Sco, and SINFONI/VLT observations secured in 2011. Results. We report the discovery of a dusty hourglass-shaped bipolar nebula. The apparent size of the nebula increased from 0.30 × 0.17 in July 2009 to 0.64 × 0.42 in July 2011. The aspect ratio suggests that the source is seen at high inclination. The central source shines efficiently in the K band and represents more than 56 ± 5% of the total flux in 2009, and 87 ± 6% in 2011. A mean expansion rate of 0.39 ± 0.03 milliarcsec per day is inferred from the VISIR observations in the direction of the major axis, which represents a projected upper limit. Assuming that the dust shell expands in that direction as fast as the low-excitation slow ejecta detected in spectroscopy, this yields a lower limit distance to V1280 Sco of ∼1 kpc; however, the systematic errors remain large due to the complex shape and velocity field of the dusty ejecta. The dust seems to reside essentially in the polar caps and no infrared flux is detected in the equatorial regions in the latest dataset. This may imply that the mass-loss was dominantly polar. Conclusions. V1280 Sco is an excellent test case for studying the temporal evolution of dusty bipolar ejecta. As the nebula expands, observations will be easier and we advocate a yearly monitoring of the source using high angular resolution techniques.

We present and analyze optical photometry and high resolution SALT spectra of the symbiotic recurrent nova V3890 Sgr at quiescence. The orbital period, P = 747.6 days has been derived from both photometric and spectroscopic data. Our double-line spectroscopic orbits indicate that the mass ratio is q = Mg/MWD = 0.78 ± 0.05, and that the component masses are MWD ≈ 1.35 ± 0.13 M⊙ and Mg ≈ 1.05 ± 0.11 M⊙. The orbit inclination is ≈67 − 69○. The red giant is filling (or nearly filling) its Roche lobe, and the distance set by its Roche lobe radius, d ≈ 9 kpc, is consistent with that resulting from the giant pulsation period. The outburst magnitude of V3890 Sgr is then very similar to those of RNe in the Large Magellanic Cloud. V3890 Sgr shows remarkable photometric and spectroscopic activity between the nova eruptions with timescales similar to those observed in the symbiotic recurrent novae T CrB and RS Oph and Z And-type symbiotic systems. The active source has a double-temperature structure which we have associated with the presence of an accretion disc. The activity would be then caused by changes in the accretion rate. We also provide evidence that V3890 Sgr contains a CO WD accreting at a high, ∼ a few× 10−8–10−7 M⊙ yr−1, rate. The WD is growing in mass, and should give rise to a Type Ia supernova within $\lesssim 10^6$ yrs - the expected lifetime of the red giant.