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We present optical multi-colour photometry of V404 Cyg during the outburstfrom December, 2015 to January, 2016 together with the simultaneous X-ray data.This outburst occurred less than 6 months after the previous outburst inJune-July, 2015. These two outbursts in 2015 were of a slow rise and rapiddecay-type and showed large-amplitude ($\sim$2 mag) and short-term ($\sim$10min-3 hours) optical variations even at low luminosity (0.01-0.1$L_{\rm Edd}$).We found correlated optical and X-ray variations in two $\sim$1 hour timeintervals and performed Bayesian time delay estimations between them. In theprevious version, the observation times of X-ray light curves were measured atthe satellite and their system of times was Terrestrial Time (TT), while thoseof optical light curves were measured at the Earth and their system of timeswas Coordinated Universal Time (UTC). In this version, we have corrected theobservation times and obtained a Bayesian estimate of an optical delay againstthe X-ray emission, which is $\sim$30 s, during those two intervals. Inaddition, the relationship between the optical and X-ray luminosity was $L_{\rmopt} \propto L_{\rm X}^{0.25-0.29}$ at that time. These features can benaturally explained by disc reprocessing.

Rapid Optical Variations Correlated with X-rays in the 2015 Second Outburst of V404 Cygni (GS 2023$+$338)