  The existence of older stars within a young star cluster can be interpreted
to imply that star formation occurs on time scales longer than a free-fall time
of a pre-cluster cloud core. Here the idea is explored that these older stars
are not related to the star formation process forming the young star cluster
but rather that the orbits of older field stars are focused by the collapsing
pre-cluster cloud core. Two effects appear: The focussing of stellar orbits
leads to an enhancement of the density of field stars in the vicinity of the
centre of the young star cluster. And due to the time-dependent potential of
the forming cluster some of these stars can get bound gravitationally to the
cluster. These stars exhibit similar kinematical properties as the newly formed
stars and can not be distinguished from them on the basis of radial-velocity or
proper-motion surveys. Such contaminations may lead to a wrong apparent
star-formation history of a young cluster. In the case of the ONC the
theoretical number of gravitationally bound older low-mass field stars agrees
with the number of observed older low-mass stars.
