Abstract
Summary Using a new numerical method and considering complete evolutionary stellar models, the linear non-adiabatic pulsations of helium stars are re-examined. A variety of new strange modes are found, some of which are unstable and extend - consistently with the observations - the instability strip to high temperatures up to log Teff = 4.3. The origin of the various modes is identified and an alternative interpretation of the strange mode phenomenon is provided. The instabilities associated with the strange modes are not due to the K mechanism and are explained - as the strange mode phenomenon itself - by mode coupling. It is demonstrated, by considering the stability of AGB envelopes, that strange modes seem to be a common feature for strong deviations from adiabaticity and a pronounced core envelope structure of the star.
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