  We study the optical evolution of the 2015 outburst in V404 Cyg, with
emphasis on the peculiar nebular phase and subsequent decay to quiescence. From
the decay timescale of the Balmer emission associated with the nebula we
measure an outflow mass M_wind~4x10^{-6} Msun. Remarkably, this is ~100 times
larger than the accreted mass and ~10% of the total mass stored in the disc.
The wind efficiency must therefore be significantly larger than previous
estimates for black hole transients, suggesting that radiation pressure (in
addition to other mechanisms such as Compton-heating) plays a key role in V404
Cyg. In addition, we compare the evolution of the 2015 and 1989 outbursts and
find clear similarities (namely a large luminosity drop ~10 d after the X-ray
trigger, followed by a brief nebular phase) but also remarkable differences in
decay timescales and long-term evolution of the Halpha profile. In particular,
we see evidence for a rapid disc contraction in 2015, consistent with a burst
of mass transfer. This could be driven by the response of the companion to hard
X-ray illumination, most notably during the last gigantic (super-Eddington)
flare on 25 June 2015. We argue that irradiation and consequential disc wind
are key factors to understand the different outburst histories in 1989 and
2015. In the latter case, radiation pressure may be responsible for the abrupt
end of the outburst through depleting inner parts of the disc, thus quenching
accretion and X-ray irradiation. We also present a refined orbital period and
updated ephemeris.
