  V3890 Sgr is a recurrent nova which has been seen in outburst three times so
far, with the most recent eruption occurring on 2019 August 27 UT. This latest
outburst was followed in detail by the Neil Gehrels Swift Observatory, from
less than a day after the eruption until the nova entered the Sun observing
constraint, with a small number of additional observations after the constraint
ended. The X-ray light-curve shows initial hard shock emission, followed by an
early start of the super-soft source phase around day 8.5, with the soft
emission ceasing by day 26. Together with the peak blackbody temperature of the
super-soft spectrum being ~100 eV, these timings suggest the white dwarf mass
to be high, ~1.3 M_sun. The UV photometric light-curve decays monotonically,
with the decay rate changing a number of times, approximately simultaneously
with variations in the X-ray emission. The UV grism spectra show both line and
continuum emission, with emission lines of N, C, Mg and O being notable. These
UV spectra are best dereddened using an SMC extinction law. Optical spectra
from SMARTS show evidence of interaction between the nova ejecta and wind from
the donor star, as well as the extended atmosphere of the red giant being
flash-ionized by the super-soft X-ray photons. Data from NICER reveal a
transient 83 s quasi-periodic oscillation, with a modulation amplitude of 5 per
cent, adding to the sample of novae which show such short variabilities during
their super-soft phase.
