The recurrent nova RS Oph: simultaneous B- and V- band observationsof the flickering variability

We performed 48.6 hr (over 28 nights) of simultaneous B- and V- band observations of the flickering variability of the recurrent nova RS Oph in quiescence. During the time of our observations, the brightness of the system varied in the range 13.2 > B > 11.1 and the colour in the range 0.86 < B - V < 1.33. We find that RS Oph becomes bluer as it becomes brighter; however, the hot component becomes redder as it becomes brighter (assuming that the red giant is non-variable). During all the runs, RS Oph exhibits flickering with amplitude 0.16-0.59 mag in the B band. We find that the flickering source has colour -0.14 < B - V < 0.40, temperature in the range 7200 < T_fl < 18 900 K, and an average radius 1.1 < R_fl < 6.7 R_⊙. We do not find a correlation between the temperature of the flickering and the brightness. However, we do find a strong correlation (correlation coefficient 0.81, significance 1.1 × 10^-7) between the B- band magnitude and the average radius of the flickering source; that is, as the brightness of the system increases, the size of the flickering source also increases. The estimated temperature is similar to that of the bright spot of cataclysmic variables. The persistent presence of flickering indicates that the white dwarf is actively accreting material for the next outburst.