  We analyze the dynamical Casimir effect for a massless scalar field confined
between two concentric spherical shells which impose on the field mixed
boundary conditions. We thus complement a previous result [Phys. Rev. A
\textbf{78}, 032521 (2008)], where the same problem was considered but in that
case the field was submitted to a Dirichlet boundary condition in both moving
spherical shells. A general expression for the average number of created
particles is deduced for an arbitrary law of radial motion of the spherical
shells. This expression is then applied to harmonic oscillations of the shells
and the number of created particles is analyzed and compared with the results
obtained under Dirichlet-Dirichlet boundary conditions.
