XTE J1118+480: Clues on the nature of the accretion flow from the optical variability

We show how the simultaneous presence of a strong quasi periodic oscillatio n (QPO) of period similar to 10 seconds in the optical and X-ray lightcurve s of the X-ray transient XTE J1118+480 can be used to obtain information ab out the nature of the accretion Mow around the source. The unusually high o ptical-to-X-ray flux ratio and the QPO observed simultaneously in both ener gy bands suggest that a significant fraction of the optical flux might orig inate close to the central source, where most of the X-rays are produced an d be indicative of a magnetically dominated corona. We also show how the te mporal evolution of the QPO can provide us with information on both the inn er radius and the viscous properties of the optically thick accretion disc.