Two distinct frequencies are often present in frequency power spectra of low-mass X-ray binary lightcurves. These frequencies are closely related to each other and there is a strong evidence that they have origin in the orbital motion in an accretion disc. A mechanism has to be in act that provides a modulation of emergent flux at observed frequencies – not necessarily the actual frequencies at which the fluid oscillates. Using numerical ray-tracing we show that strong gravity has an effect on light modulation and may itself be responsible for significant part of variations in observed lightcurves.
