  For a fixed $j$-invariant $j_0$ of an elliptic curve without complex
multiplication we bound the number of $j$-invariants $j$ that are algebraic
units and such that elliptic curves corresponding to $j$ and $j_0$ are
isogenous. Our bounds are effective. We also modify the problem slightly by
fixing a singular modulus $\alpha$ and looking at all $j$ such that $j-\alpha$
is an algebraic unit and such that elliptic curves corresponding to $j$ and
$j_0$ are isogenous. The number of such $j$ can again be bounded effectively.
