  Let $s(n)$ be the number of nonzero bits in the binary digital expansion of
the integer $n$. We study, for fixed $k,\ell,m$, the Diophantine system $$
  s(ab)=k, \quad s(a)=\ell,\quad \mbox{and }\quad s(b)=m, $$ in odd integer
variables $a,b$. When $k=2$ or $k=3$, we establish a bound on $ab$ in terms of
$\ell$ and $m$. While such a bound does not exist in the case of $k=4$, we give
an upper bound for $\min\{a,b\}$ in terms of $\ell$ and $m$.
