  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.
