  The black-hole binary, V404 Cygni, went into outburst in June 2015, after 26
years of X-ray quiescence. We observed the outburst with the Neil Gehrels Swift
observatory. We present optical/UV observations taken with the Swift
Ultra-violet Optical Telescope, and compare them with the X-ray observations
obtained with the Swift X-ray Telescope. We find that dust extinction affecting
the optical/UV, does not correlate with absorption due to neutral hydrogen that
affects the X-ray emission. We suggest there is a small inhomogeneous high
density absorber containing a negligible amount of dust, close to the black
hole. Overall, temporal variations in the optical/UV appear to trace those in
the X-rays. During some epochs we observe an optical time-lag of (15 - 35)s.
For both the optical/UV and X-rays, the amplitude of the variations correlates
with flux, but this correlation is less significant in the optical/UV. The
variability in the light curves may be produced by a complex combination of
processes. Some of the X-ray variability may be due to the presence of a local,
inhomogeneous and dust-free absorber, while variability visible in both the
X-ray and optical/UV may instead be driven by the accretion flow: the X-rays
are produced in the inner accretion disc, some of which are reprocessed to the
optical/UV; and/or the X-ray and optical/UV emission is produced within the
jet.
