WWFI u′g′r′ color composition of the ONC (image credit: Wendelstein Observatory). Overlaid are the stars monitored in all three bands. The yellow symbols indicate stars that have a match with the X-ray sources listed in the COUP catalog (J/ApJS/160/319/coup, Getman et al. 2005). The star symbols further indicate sources that were monitored for at least five epochs and allowed an estimate for the mass accretion rate.