  We show how the simultaneous presence of a strong quasi periodic oscillation
(QPO) of period of about 10 seconds in the optical and X-ray lightcurves of the
X-ray transient XTE J1118+480 can be used to obtain information about the
nature of the accretion flow around the source. The unusually high
optical-to-X-ray flux ratio and the QPO observed simultaneously in both energy
bands suggest that a significant fraction of the optical flux might originate
close to the central source, where most of the X-rays are produced and be
indicative of a magnetically dominated corona. We also show how the temporal
evolution of the QPO can provide us with information on both the inner radius
and the viscous properties of the optically thick accretion disc.
