Abstract
A CCD photometry of the dwarf nova MASTER OT J172758.09 +380021.5 was carried out in 2019 during 134 nights. Observations covered three superoutbursts, five normal outbursts and quiescence between them. The available ASASSN and ZTF data for 2014-2020 were also examined. Spectral observations were done in 2020 when the object was in quiescence. Spectra and photometry revealed that the star is an H-rich active ER UMa-type dwarf nova with a highly variable supercycle of ~50-100 d that implies a high and variable mass-transfer rate. This object demonstrated peculiar behaviour: short-lasted superoutbursts (a week); a slow superoutburst decline and cases of rebrightenings; low frequency (from none to a few) of the normal outbursts during the supercycle. In 2019 a mean period of positive superhumps was found to be 0.05829 d during the superoutbursts. Late superhumps with a mean period of 0.057915 d which lasted about ~20 d after the end of superoutburst and were replaced by an orbital period of 0.057026 d or its orbital-negative superhump beat period were detected. An absence of eclipse in the orbital light curve and its moderate amplitude are consistent with the orbital inclination of about 40 degr found from spectroscopy. The blue peaks of the V-Ic and B-Rc of superhumps during the superoutburst coincided with minima of the light curves, while B-Rc of the late superhumps coincided with a rising branch of the light curves. We found that a low mass ratio q=0.08 could explain most of the peculiarities of this dwarf nova. The mass-transfer rate should be accordingly higher than what is expected from gravitational radiation only, this assumes the object is in a post-nova state and underwent a nova eruption relatively recently -- hundreds of years ago. This object would provide probably the first observational evidence that a nova eruption can occur even in CVs near the period minimum.
Highlights
Cataclysmic variables (CVs) are close binary systems in which a white dwarf (WD) accretes matter from a late-type donor star that fills its Roche lobe
This result is in a good agreement with the estimate for the mass described in Section 3 and places MASTER 1727 near a low-mass bound of the H-rich SU UMa stars (Fig. 13)
No new SU UMa type-stars were known between these two groups
Summary
Cataclysmic variables (CVs) are close binary systems in which a white dwarf (WD) accretes matter from a late-type donor star that fills its Roche lobe. During the superoutbursts periodic brightness variations (positive superhumps) with periods of a few percent longer than the orbital period are present These superhumps, as it was shown by Whitehurst (1988); Osaki (1989); Hirose & Osaki (1990); Lubow (1991), are the consequence of tidal instability resulting from the 3:1 resonance in the accretion disc. Osaki & Kato (2014) suggested that decrease of the frequency of the normal outbursts in some DNe resulting in a transition from a short (S) cycle to a long (L) one (designations of cycles, i.e., the interval between two successive outbursts, is taken from Smak (1985)) could be caused by a transition of a disc from coplanar with an orbital plane to a tilted to the orbital plane and the subsequent appearance of nodal precession and negative superhumps. We present a spectral and photometric study of MASTER 1727 at different stages of its activity based on our observations in 2019 – 2020 and on the ASASSN and ZTF3 (Masci et al, 2019) database in 2014 – 2019
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