  Let $F$ be an irreducible binary form attached to a number field $K$ of
degree $\geq 3$. Let $\epsilon\not\in \{-1, 1\}$ be a totally real unit of $K$.
By twisting $F$ with the powers $\epsilon^a$ of $\epsilon$, ($a\in{\mathbf
Z}$), we obtain an infinite family $F_a$ of binary forms. Let $m\in{\mathbf
Z}$. We give an effective bound for $\max\{|a|, \log|x|, \log|y|\}$ when
$a,x,y$ are rational integers satisfying $F_a(x,y)=m$ with $xy\not=0$.
