Heat kernel coefficients for the dielectric cylinder

We calculate the heat kernel coefficients for the electromagnetic field in the background of a dielectric cylinder with nonequal speeds of light inside and outside. The coefficient $a_2$ whose vanishing makes the vacuum energy of a massless field unique, turns out to be zero in dilute order, i.e., in order $(\varepsilon -{1)}^2$, and nonzero beyond. As a consequence, the vanishing of the vacuum energy in the presence of a dielectric cylinder found by the Casimir-Polder summation must take place irrespective of the methods by which it might be calculated.