  The $\alpha$--$\alpha$ differential cross sections are analyzed in the
optical model using a double--folded potential. With the knowledge of this
potential bound and resonance--state properties of $\alpha$--cluster states in
$^{8}$Be and $^{12}$C as well as astrophysical S--factors of
$^{4}$He($\alpha$,$\gamma$)$^{8}$Be and $^{8}$Be($\alpha$,$\gamma$)$^{12}$C are
calculated. $\Gamma_{\gamma}$--widths and B(E2)--values are deduced.
