  The existence of trapped modes in coupled electromagnetic waveguides is
experimentally investigated for configurations with different degrees of
symmetry supporting hybrid modes. The occurrence of confined solutions in such
open geometries is proven to be much more general than demonstrated so far, as
predicted by Goldstone and Jaffe [J. Goldstone and R. L. Jaffe, Phys. Rev. B
45, 14100 (1992)]. The identification of the observed modes is based on the
numerical modeling of the relative vector fields. The experimental results
evidence as an increasing aperture of the configuration can improve the
confinement of the mode instead of generating additional leakage channels. In
particular, long-lived trapped modes can be easily obtained in geometries with
a relevant degree of aperture. Their resonance parameters can compete with
those of standard close cavities working at millimeter wavelengths. The role of
the symmetry on the properties of these trapped modes is discussed in detail.
