  A test of Lorentz invariance for electromagnetic waves was performed by
comparing the resonance frequencies of two stable optical resonators as a
function of orientation in space. The crystalline resonators were operated at
3.4 K in a cryostat employing a pulse-tube refrigerator. A new analysis yields
the Robertson-Mansouri-Sexl theory parameter combination
$\beta-\delta-1/2=(-0.6\pm 2.1\pm1.2)\cdot10^{-10}$ and one parameter of the
Standard Model Extension theory, $(\tilde{\kappa}_{e-})^{ZZ} =
(-2.9\pm2.2)\cdot 10^{-14}$.
