  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
($8M_\odot - 90M_\odot$). 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 $2\times10^5$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 $60M_\odot$ 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.
