  We consider the $t$-core of an $s$-core partition, when $s$ and $t$ are
coprime positive integers. Olsson has shown that the $t$-core of an $s$-core is
again an $s$-core, and we examine certain actions of the affine symmetric group
on $s$-cores which preserve the $t$-core of an $s$-core. Along the way, we give
a new proof of Olsson's result. We also give a new proof of a result of
Vandehey, showing that there is a simultaneous $s$- and $t$-core which contains
all others.
