Consider a rational point on an elliptic curve under an isogeny. Suppose that
the action of Galois partitions the set of its pre-images into n orbits. It is
shown that all such points above a certain height have there denominator
divisible by n distinct primes. This generalizes Siegel's theorem and more
recent results of Everest et al. For multiplication by a prime l, it is shown
that if n>1 then either the point is l times a rational point or the elliptic
curve emits a rational l-isogeny.