the first overtone are unstable with the ratio of periods $P_2/P_1 \simeq 0.87$. The most interesting behaviour of $\eta$ is visible in the vicinity of the first maximum. There is a clearly visible pattern of consecutive local minima and maxima, which does not occur in MS models. This phenomenon is related to the partial trapping of pulsation modes in the radiative core and the envelope. The detailed description of this phenomenon can be found in our previous paper (D2013).

In Model2, which burns helium in its core, a situation is different. There are only a few very high-order low-frequency g-modes which are unstable. These maximum values of $\eta$ are shifted towards lower frequencies when compared to Model1. There is no pattern of local minima and maxima and the calculated frequency spectrum is much sparser than for Model1. This is typical for every blue-loop that a number of unstable modes is very low. Model2 has one unstable dipole mode with frequencies of $\nu\approx 0.08$ c/d and two unstable quadruple modes with frequencies $\nu\approx 0.14,~0.15$ c/d. Radial modes are always stable in models during core helium burning phase, regardless of metallicity, opacity tables, etc. However, the ratio of periods between the first 