Non-LTE strengths of Si II lines were computed in an effort to reproduce the observed strengths of Si II lines in the shell spectrum of ζ Tau. This effort was unsuccessful, probably because it assumed that the shell could be represented by a finite, plane-parallel slab. The main conclusion of this study, then, is that in order to understand the shell spectra of Be stars, a more realistic geometry must be assumed, in which the shell is represented by a thick disk.

Radial velocities and profiles of the components of the H and K lines in the spectrum of HD 190073 are analyzed in a 24-spectrum sample covering the period 1943–74. A 2-to-1 ratio in the radial velocities of some components is shown not to be significant on a spectrum-to-spectrum basis, contrary to suggestions by Merrill (1951) and Scargle (1973). The details of the H and K complex structure are correlated with the profiles of the Balmer lines. Radiative forces acting selectively via a resonance scattering mechanism of Ca+ atoms are capable of producing the main features of the complex profiles of H and K in HD 190073 and in other stars exhibiting similarities to HD 190073 (such as P Cygni Balmer lines and infrared excess).

A rotationally symmetrical disk-shaped envelope was considered and the steady-state equations for level populations solved using the escape probability approximation. The line profiles were then calculated and compared with observations.

The stars V923 Aql (HD 183656) and EW Lac (HD 217050) are marked in the General Catalog of Variable Stars (Kukarkin et al., 1969) as unique, similar objects with shells. The two stars have quasiperiodic variations in brightness (Lynds, 1960; Walker, 1953). It is assumed here that the variation of brightness is caused by an inhomogeneous distribution of brightness over the surface of the star. A consideration of the literature data on the radial velocities Vr which we have carried out has shown that the variation of Vr is different (Figure 1).

A survey is presented of the theoretical attempts to determine the structure of the circumstellar matter around Be stars. The general equations describing the structure and dynamics of Be star envelopes are given. The complications introduced by various physical phenomena are briefly discussed and initial attempts to solve restricted problems are considered. The various ad hoc models proposed for Be stars are discussed and comparisons of the observations with predictions of these models are illustrated. The strengths and weaknesses of these models are evaluated and areas where progress is being or should be made are considered.

A problem that has intrigued me for some years is that of the dynamics of the Be phenomenon. For, although the Be phenomenon has been extensively studied for decades, the problems associated with the dynamics of the circumstellar envelopes about Be stars - the problems associated with their formation, with their maintenance, and with their dissolution - have, until very recently, received little more than qualitative consideration.

We have examined the theoretical wavelength dependence of linear polarization produced by electron scattering and modified by an absorptive opacity in the extended, distorted atmospheres of Be and Wolf-Rayet stars. A model atmosphere representing a Wolf-Rayet star recently calculated by Cassinelli and Hartmann was used. This model takes into consideration the subsonic portions of a radial, steady-state flow together with the requirement of radiative equilibrium. The model was then scaled down in temperature to approximate the cooler Be stars, again assuming radiative equilibrium. There were four models altogether, one WR star, and three Be models. All the atmospheres are quite extended by radiation pressure.

A detailed calculation of the radiative acceleration in B-type stars shows it to be a double-peaked function of effective temperature at small optical depths. The two peaks are shown to coincide approximately with peaks in the distribution of mean Hα emission strength as a function of B - V color in Be stars. These facts suggest that radiation may play an important role in the support of the Be star extended atmosphere.

Observed Be stars are considered to be interacting binaries undergoing mostly case B mass exchange. Their observed properties can be explained satisfactorily as the consequences of different modes of mass transfer. Suggestions for further observational and theoretical studies to verify or to disprove the hypothesis presented here are briefly outlined.

Possible methods for detecting the presence of a cool companion to a Be star are discussed. Photometric observations are shown to be incapable of detecting companions in all but the most extreme cases. Spectroscopic investigation is also unlikely to yield many new discoveries. It, however, remains the most promising method for the detection of binary Be stars. The four known binary Be stars are also discussed.

Infrared calcium triplet emission in Be stars is discussed in detail. The lines are shown to originate in a region of large optical thickness and low temperature (T ~ 5000 K). The possible connection between the presence of calcium triplet emission and binary nature is briefly discussed.

In this paper, we discuss the Be stars which show recurrent shell structure and/or cyclic variations in the profiles of their emission lines with periods near 100d and, thus, are good candidates for interacting binaries undergoing mass-exchange. The periodic spectral variations observed in the Ble star HR 2142 are reviewed and brought up to date in the light of recent observations. HR 2142 is then compared with other periodic binary Be stars (AX Mon, 17 Lep, HD 218393, HD 173219, and φ Per) in order to point out the wide variance in observed spectra which interacting binaries can display.

Arguments are presented against the adoption of an early B-red giant combination for the Φ Persei system. Velocity curves of the primary and secondary components based on the measures of the He I lines on nearly two hundred plates are shown and discussed insofar as orbital parameters and system membership are concerned. Evidence from the He I lines, the emission edges, shell absorptions, and underlying wide absorptions of the hydrogen lines indicate that the system is made up of a B1 primary and a B3 secondary, both of which are emission-line stars.

The hypothesis that a significant fraction of Be stars are interacting binaries is discussed in general terms. The author believes that the observed Be stars are a mixture of three different types of objects. The weakest point of the single-star hypothesis is probably the difficulty in explaining the quasi-periodic V/R variations; the weakest point of the binary hypothesis appears to be the statistical expectancy of a fairly high number of eclipsing systems among them.