(A) The possibilities and the difficulties of a theoretical study of extended atmospheres in binaries are briefly discussed.(B) We try to summarize and discuss critically the present status of the theory of three types of extended atmospheres (i.e. atmospheres in which the average photon mean-free-path is the same order of magnitude or larger than the stellar radius):(1) Extended atmospheres in hydrostatic and in grey or non-grey radiative equilibrium.(2) Dynamic (expanding) atmospheres which occur if the radiative acceleration is slightly smaller than the acceleration of gravity.(3) Stellar coronae which are formed in the presence of a mechanical energy flux.In (1) we study the importance of the ‘forward peaking’ of the radiation field in the outer layers of the atmosphere. The possibilities for the solution of the non-grey transfer problem in an extended atmosphere are discussed.In (2) we pay special attention to Marlborough's and Roy's (1970) result that the atmospheric gas cannot be accelerated directly to supersonic velocities by the action of the radiation force.In (3) the large differences in the coronal properties of stars of different chemical composition are emphasized. We draw attention to the partially unexplored but probably very interesting properties of coronae of helium-rich stars.
The Theory of Extended and Expanding Atmospheres