  We present a theoretical study of the $\gamma\gamma \to \pi\eta$ process from
the threshold up to 1.4 GeV in the $\pi\eta$ invariant mass. For the s-wave
$a_0(980)$ resonance state we adopt a dispersive formalism using a
coupled-channel Omn\`es representation, while the d-wave $a_2(1320)$ state is
described as a Breit-Wigner resonance. An analytic continuation to the
$a_0(980)$ pole position allows us to extract its two-photon decay width as
$\Gamma_{a_0\to\gamma\gamma}=0.27(4)$ keV.
