  Let $\L$ be an artin algebra. Iyama conjectures that the endomorphism ring of
any two maximal $l$-orthogonal modules, $M_1$ and $M_2$, are derived
equivalent. He proves the conjecture for $l=1$, and for $l>1$ he gives some
orthogonality condition on $M_1$ and $M_2$, such that the
$\End_\L(M_2)^\op$-$\End_\L(M_1)$-bimodule $\Hom_\L(M_2,M_1)$ is tilting, which
implies that the rings $\End_\L(M_2)$ and $\End_\L(M_1)$ are derived equivalent
(see \cite{H}). The purpose of this paper is to characterize tilting modules of
the form $\Hom_\L(M_2,M_1)$ in terms of the relative theories induced by the
$\L$-modules $M_1$ and $M_2$, thus getting a generilization of Iyama's result.
