According to studies of properties and evolution of protoplanetary discs in young clusters (e.g. Hillenbrand 2005; Sicilia-Aguilar et al. 2006), it is valid to assume that the protoplanetary discs in the ONC have been only marginally subject to internal disc processes. As such, we assume in the following that the effect of external processes on discs, i.e. photevaporation and encounters, have not been masked by internal processes at the current age of the ONC.

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The time scale of the potential variation is related to the crossing time of the cluster, $t_{\rm cross} \approx 0.5$ Myr, while the time scale of revolution is approximately the cluster age, $t_{\rm ONC} \approx 1$ Myr, which is about twice as large.

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