  We present a method of determining the CKM-angle $\alpha$ by performing
simultaneous measurements of the mixing-induced CP asymmetries of the decays
$B_d\to\pi^+\pi^-$ and $B_d\to K^0\bar K^0$. The accuracy of our approach is
limited by $SU(3)$-breaking effects originating from $\bar b\to \bar ds\bar s$
QCD-penguin diagrams. Using plausible power-counting arguments we show that
these uncertainties are expected to be of the same order as those arising
through electroweak penguins in the standard Gronau-London-method in which
$\alpha$ is extracted by means of isospin relations among $B\to\pi\pi$ decay
amplitudes. Therefore our approach, which does not involve the experimentally
difficult mode $B_d\to\pi^0\pi^0$ and is essentially unaffected by electroweak
penguins, may be an interesting alternative to determine $\alpha$.
