  Context. We investigate small-amplitude light variations in FU Ori occurring
in timescales of days and weeks.
  Aims. We seek to determine the mechanisms that lead to these light changes.
  Methods. The visual light curve of FU Ori gathered by the MOST satellite
continuously for 55 days in the 2013-2014 winter season and simultaneously
obtained ground-based multi-colour data were compared with the results from a
disc and star light synthesis model.
  Results. Hotspots on the star are not responsible for the majority of
observed light variations. Instead, we found that the long periodic family of
10.5-11.4 d (presumably) quasi-periods showing light variations up to 0.07 mag
may arise owing to the rotational revolution of disc inhomogeneities located
between 16-20 solar radii. The same distance is obtained by assuming that these
light variations arise because of a purely Keplerian revolution of these
inhomogeneities for a stellar mass of 0.7 solar mass. The short-periodic
(3-1.38 d) small amplitude (0.01 mag) light variations show a clear sign of
period shortening, similar to what was discovered in the first MOST
observations of FU Ori. Our data indicate that these short-periodic
oscillations may arise because of changing visibility of plasma tongues (not
included in our model), revolving in the magnetospheric gap and/or likely
related hotspots as well.
  Conclusions. Results obtained for the long-periodic 10-11 d family of light
variations appear to be roughly in line with the colour-period relation, which
assumes that longer periods are produced by more external and cooler parts of
the disc. Coordinated observations in a broad spectral range are still
necessary to fully understand the nature of the short-periodic 1-3 d family of
light variations and their period changes.
