  We study the simultaneous influence of boundary conditions and external
fields on quantum fluctuations by considering vacuum zero-point energies for
quantum fields in the presence of a magnetic fluxon confined by a bag, circular
and spherical for bosons and circular for fermions. The Casimir effect is
calculated in a generalized cut-off regularization after applying zeta-function
techniques to eigenmode sums and using recent techniques about Bessel zeta
functions at negative arguments.
