Axion-photon mixing has been proposed as an alternative to acceleration as the explanation for supernovae dimming .
We point out that the loss of photons due to this mixing will induce a strong asymmetry between the luminosity , d _ { L } ( z ) , and angular-diameter distance , d _ { A } ( z ) , since the latter is unaffected by mixing .
In a first search for such an asymmetry we introduce a dimensionless mixing amplitude \lambda so that \lambda = 0 if no photons are lost and \lambda \approx 1 if axion-photon mixing occurs .
The best-fit to SNIa and radio galaxy data is \lambda = -0.3 ^ { +0.6 } _ { -0.4 } ( 95 % CL ) , corresponding to an unphysical , negative , mixing length .
This same argument limits the attenuation of light from supernovae due to dust .
We show that future d _ { L } and d _ { A } data from SNAP and galaxy surveys such as DEEP2 and KAOS will detect or rule out mixing at more than 5 \sigma , almost independently of the dark energy dynamics .
Finally we discuss the constraints from the near maximal polarisation of the gamma-ray burst ( GRB ) GRB021206 .
Since mixing reduces the polarisation of distant sources , future observations of high redshift GRBs may provide orthogonal constraints on axion-photon mixing and related scenarios .