In order to understand the periodic and semi-periodic variations of luminous O- B- A-type stars , linear nonadiabatic stability analyses for radial and nonradial oscillations have been performed for massive evolutionary models ( 8 M _ { \odot } -90 M _ { \odot } ) .
In addition to radial and nonradial oscillations excited by the kappa-mechanism and strange-mode instability , we discuss the importance of low-degree oscillatory convection ( nonadiabatic g ^ { - } ) modes .
Although their kinetic energy is largely confined to the convection zone generated by the Fe opacity peak near 2 \times 10 ^ { 5 } K , the amplitude can emerge to the photosphere and should be observable in a certain effective temperature range .
They have periods longer than those of the radial strange modes so that they seem to be responsible for some of the long-period microvariations of LBVs ( S Dor variables ) and \alpha Cyg variables .
Moreover , monotonously unstable radial modes are found in some models whose initial masses are greater than or equal to 60 M _ { \odot } with Z = 0.02 .
The monotonous instability probably corresponds to the presence of an optically thick wind .
The instability boundary roughly coincides with the Humphreys-Davidson limit .