  The understanding of the accretion process has a central role in the
understanding of star and planet formation. We aim to test how accretion
variability influences previous correlation analyses of the relation between
X-ray activity and accretion rates, which is important for understanding the
evolution of circumstellar disks and disk photoevaporation. We monitored
accreting stars in the Orion Nebula Cluster from November 24, 2014, until
February 17, 2019, for 42 epochs with the Wendelstein Wide Field Imager in the
Sloan Digital Sky Survey u'g'r' filters on the 2 m Fraunhofer Telescope on
Mount Wendelstein. Mass accretion rates were determined from the measured
ultraviolet excess. The influence of the mass accretion rate variability on the
relation between X-ray luminosities and mass accretion rates was analyzed
statistically. We find a typical interquartile range of ~ 0.3 dex for the mass
accretion rate variability on timescales from weeks to ~ 2 years. The
variability has likely no significant influence on a correlation analysis of
the X-ray luminosity and the mass accretion rate observed at different times
when the sample size is large enough. The observed anticorrelation between the
X-ray luminosity and the mass accretion rate predicted by models of
photoevaporation-starved accretion is likely not due to a bias introduced by
different observing times.
