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We present high time‐resolution multicolour optical observations of the quiescent X‐ray transients GRS 1124–684 (=GU Mus) and Cen X‐4 (=V822 Cen) obtained with ULTRACAM. Superimposed on the secondary stars' ellipsoidal modulation in both objects are large flares on time‐scales of 30–60 min as well as several distinct rapid flares on time‐scales of a few minutes, most of which show further variability and unresolved structure. Not significant quasi‐periodic oscillations are observed and the power density spectra of GRS 1124–684 and Cen X‐4 can be described by a power law. From the colour–colour diagrams of the flare events, for GRS 1124–684 we find that the flares can be described by hydrogen gas with a density of   N  H ∼ 10 24  nucleons cm −2  , a temperature of ∼8000 K and arising from a radius of  ∼0.3 R ⊙  . Finally we compile the values for the transition radius (the radius of the hot advection‐dominated accretion flow) estimated from quasi‐periodic oscillations and/or breaks in the power density spectrum for a variety of X‐ray transients in different X‐ray states. As expected, we find a strong correlation between the bolometric luminosity and the transition radius.

Observations of the quiescent X‐ray transients GRS 1124–684 (=GU Mus) and Cen X‐4 (=V822 Cen) taken with ULTRACAM on the VLT ★