  Hybrid stars like Nu Eridani and 12 Lacertae show two different types of
pulsations: (i) low-order acoustic and gravity modes of the Beta Cephei type
with periods of about 3-6 hours, and (ii) high-order gravity modes of the SPB
type with periods of about 1.5-3 days. Theoretical computations using both OPAL
and OP opacity data well reproduce short period low-order pulsations of the
Beta Cep type and show a tendency to instability of high-order gravity modes,
especially for stellar models built with the OP opacities. However, instability
at observed long periods has not been achieved. We test effects of artificial
opacity modifications in the deep envelope on the instability of some hybrid
star models. For Nu Eri models, an opacity increase both in the Z opacity bump
region at temperature of about 200 000 K and in the region of the deeper
opacity bump at temperature of about 2-2.5 million degrees (this bump is also
mainly due to excited ions of the iron-group elements) may result in
instability of the high-order gravity modes with the observed periods. The
shortest observed period of 3 hours can also be excited in modified models.
However, the required opacity increase seems to be quite large (up to 2 times
in some stellar layers) which may be incompatible with atomic physics.
