  We study vector fields on a disk satisfying two types of mixed boundary
conditions. These boundary conditions are selected by BRST-invariance in
electrodynamics. They also appear in the de Rham complex. The manifest
construction of the harmonic expansion is presented. The eigenfunctions of the
vector Laplace operator are expressed in terms of fields satisfying pure
Dirichlet or Robin boundary conditions. For the case of four-dimensional disk
several first coefficients of the heat kernel expansion are computed. An error
in the analitical expression by Branson and Gilkey is corrected.
