  The X-ray emission from X-ray binaries may originate in flares occurring when
magnetic loops anchored in the disc reconnect. In analogy with our Sun, H alpha
emission should arise as the accelerated electrons thermalize in the optically
emitting disc, perhaps leading to correlated variability between X-rays, H
alpha and the optical continuum. We present simultaneous X-ray and optical high
speed photometry of the neutron star low-mass X-ray binary Cyg X-2 to search
for such correlations. The highest time resolution achieved is 5 ms in white
light and 100 ms with a 3 nm filter centred on H alpha. We find power on
timescales > 100s (flickering) in optical with a total r.m.s. of a few %, about
an order of magnitude less than that seen in X-rays. We do not find significant
correlations between the X-ray and optical fluxes on short timescales, hence
cannot conclude whether magnetic flares contribute significantly to the optical
emission.
