Context. One way to understand the still mysterious Be phenomenon is to study the time variations of particular Be stars with a long observational history. ζ Tau is one obvious candidate.Aims. Using our rich series of spectral and photometric observations and a critical compilation of available radial velocities, spectrophotometry of Hα, and photometry, we characterize the pattern of time variations of ζ Tau over about a century. Our goal is to find the true timescales of its variability and confront them with the existing models related to various aspects of the Be phenomenon.Methods. Spectral reductions were carried out using the IRAF and SPEFO programs. The HEC22 program was used for both photometric reductions and transformations to . Orbital solutions were derived with the latest publicly available version of the program FOTEL, period analyses employed both the PDM and Fourier techniques – programs HEC27 and PERIOD04.Results. We derived a new orbital ephemeris HJD (2447025.6±1.8) + (132987 ±0050) . The analysis of long-term spectral and light variations shows a clear correlation between the RV and changes, and a very complex behaviour of the light changes. The character of the orbital light and changes varies from season to season.

Context. One way to understand the still mysterious Be phenomenon is to study the time variations of particular Be stars with a long observational history. ζ  Tau is one obvious candidate. Aims. Using our rich series of spectral and photometric observations and a critical compilation of available radial velocities, spectrophotometry of H α , and $U\!B{}V$ photometry, we characterize the pattern of time variations of ζ  Tau over about a century. Our goal is to find the true timescales of its variability and confront them with the existing models related to various aspects of the Be phenomenon. Methods. Spectral reductions were carried out using the IRAF and SPEFO programs. The HEC22 program was used for both photometric reductions and transformations to $U\!B{}V$. Orbital solutions were derived with the latest publicly available version of the program FOTEL, period analyses employed both the PDM and Fourier techniques – programs HEC27 and PERIOD04. Results. We derived a new orbital ephemeris $T_{\rm RV max.}=$ HJD (2447025.6 ± 1.8) + (132$^{\rm d}\!\!.$987 ± 0$^{\rm d}\!\!.$050) $\times E$. The analysis of long-term spectral and light variations shows a clear correlation between the RV and $V/R$ changes, and a very complex behaviour of the light changes. The character of the orbital light and $V/R$ changes varies from season to season.

As part of our search for the origin of stellar-wind variability, we have conducted simultaneous ultraviolet and optical spectroscopy of a few bright 0 stars. Just prior to the development of a discrete absorption component (DAC) in UV P Cygni lines an extra emission component appears in the Ha line. This emission component disappears rapidly and is followed by an increase in blue-shifted absorption. The changes in the Ha line take place at low velocity (0 — 0.21/00) on a timescale equal to the recurrence timescale of the DACs. The close coincidence in time of the observed variations shows that the wind variability, which is most prominently observed in the form of DACs at high supersonic velocity (near v^,), is also present in the Ha forming region, which is presumably close to the star. These observations and the fact that the timescale of the variations is comparable to the rotation period of the star suggest that a stellar magnetic field might play an important role in controlling the base of the stellar wind.

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f ? pe-O8fp). The Balmer and He i lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He ii absorptions and metal lines (including many selective emission lines but excluding He ii lambda 4686 angstrom emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with P-orb = 1542 d, e = 0.45. We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a similar to O8 giant with a similar to B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.

A study of the B star population of the open cluster NGC2169 through the combination of a variety of observational techniques raises interesting questions about the relationship between beta Cephei stars and Be stars, illustrating the necessity of spectroscopic observations to resolve anibiguities.

Recent near-infrared measurements of the angular diameter of Achernar (the bright Be star alpha Eridani) with the ESO VLT interferometer have been interpreted as the detection of an extremely oblate photosphere, with a ratio of equatorial to polar radius of at least 1.56 +/- 0.05 and a minor axis orientation of 39 degrees +/- 1 degrees (from North to East). The optical linear polarization of this star during an emission phase in 1995 September was 0.12 +/- 0.02% at position angle 37 degrees +/- 8 degrees (in equatorial coordinates), which is the direction of the projection of the rotation axis on the plane of the sky according to the theory of polarization by electron scattering in an equatorially flattened circumstellar disk. These two independent, determinations of the orientation of the rotation axis are therefore in agreement. The observational history of correlations between Ha emission and polarization as found in the literature is that of a typical Be star, with the exception of an interesting question raised by the contrast between Schroder's measurement of a small polarization perpendicular to the projected rotation axis in 1969-70 and Tinbergen's measurement of zero polarization in 1974.5, both at times when emission was reportedly absent.

We present the results of recent multicolor photometric and high-resolution spectroscopic observations of the bright Be star π Aqr. Observational data collected from the literature were used to study the star's variations over the last four decades. The star is identified with the IR sources F22227+0107 in the IRAS Faint Point Source Catalog and MSX5 G066.0066-44.7392 in the MSX Catalog. The variations in near-IR brightness of π Aqr are found to be among the largest reported for Be stars. Since 1996, the star has shown only weak signs of circumstellar emission, which has allowed us to refine the fundamental stellar parameters: AV = 0.15 mag, Teff = 24,000 K, log g = 3.9, and MV = -2.95 mag. A weak emission component of the Hα line has been detected during the recent quasi-normal star phase. From analysis of the Hα line profiles, we find antiphased radial velocity variations of the emission component and the photospheric absorption, with a period of 84.1 days and semiamplitudes of 101.4 and 16.7 km s-1, respectively. This result suggests that π Aqr may be a binary system consisting of stars with masses of M1 i = 12.4 M☉, M2 i = 2.0 M☉. We also estimate the orbital inclination angle to be between 50° and 75°. We suggest that the photometric, spectroscopic, and polarimetric variations observed during the second half of the 20th century may be due to variable mass transfer between the binary components.

Polarimetric data associated with multi-parameter observational campaigns of selected bright O-type stars and their variable winds, are analysed in relation to the outcomes of the UV and optical spectroscopic studies. For the stars xi Per and lambda Cep, individual measurement uncertainties are Deltap similar to +/- 0.0002 with nightly mean uncertainties of Deltap similar to +/- 0.00007. Although variability is apparent on a night-to-night basis, with differences in deltap similar to 0.0002, no correlations are found between these and the periodic behaviours associated with the stellar Si IV and Halpha lines. Similar polarimetric variability is seen in the data for the standard star phi Cas used as a reference in this observing campaign. It is suggested that all of these low level fluctuations are not intrinsic to the stars but are engendered by structured instrumental polarization in the diffraction pattern and depolarization effects in combination with inconsistent target acquisition and with variable seeing conditions in the Earth's atmosphere. Reassessment of older data for lambda Cep from Hayes (1978) also supports this thesis.

This paper describes a practical model for the polarization of Be stars that can be used to estimate roughly the physical parameters for optically thin circumstellar envelopes from broadband UBVRI photopolarimetry data. Analysis of long-term variability in terms of these parameters is a promising approach toward understanding the Be phenomenon. An interesting result from fitting the model to observations of eight Be stars is that all of them may have geometrically thin disks, with opening half-angles on the order of 10° or less. This contributes to the growing evidence that most Be disks are geometrically thin.

The Be shell star EW Lac was observed in September 1993 during a multi-site campaign. Results from visual spectroscopy and polarimetry are summarized here. He I 6678 profiles have been compared to previous observations held in 1989 and show an additional complex and highly variable circumstellar component which can due to material expelled from the star just prior to these observations. Two groups of frequencies are found again in 1993 observations compared with 1989 ones. In the frame of nrp, they could be associated to low degree g-modes.

New observations of nine of the brightest northern O stars have been made with the Breger polarimeter on the 0.9 m telescope at McDonald Observatory and the AnyPol polarimeter on the 0.4 m telescope at Limber Observatory, using the Johnson-Cousins UBVRI broadband filter system. Comparison with earlier measurements shows no clearly defined long-term polarization variability. For all nine stars the wavelength dependence of the degree of polarization in the optical range can be fitted by a normal interstellar polarization law. The polarization position angles are practically constant with wavelength and are consistent with those of neighboring stars. Thus the simplest conclusion is that the polarization of all the program stars is primarily interstellar. The O stars chosen for this study are generally known from ultraviolet and optical spectroscopy to have substantial mass loss rates and variable winds, as well as occasional circumstellar emission. Their lack of intrinsic polarization in comparison with the similar Be stars may be explained by the dominance of radiation as a wind driving force due to higher luminosity, which results in lower density and less rotational flattening in the electron scattering inner envelopes where the polarization is produced. However, time series of polarization measurements taken simultaneously with Hα and UV spectroscopy during several coordinated multiwavelength campaigns suggest two cases of possible small-amplitude, periodic short-term polarization variability, and therefore intrinsic polarization, which may be correlated with the more widely recognized spectroscopic variations.

Okazaki (1991) and Papaloizou, Savonije, & Henrichs (1992) suggested that the quasi-cyclic VIR variability observed in the emission line profiles of many Be stars is caused by a precessing one-armed density wave in the circumstellar disk. It seems likely that the changing aspect of such a non-axisymmetric density pattern might also lead to a related variation of the continuum polarization. We have searched for such an effect in two well-studied Be shell stars, zeta Tau and 48 Lib, based on data compiled from several groups of observers from 1984 to 1998. Using the Monte Carlo radiation transfer code of Wood, Bjorkman, Whitney, & Code (1998), we have calculated the polarization due to electron scattering in Be disks in the presence of one-armed density perturbations. Although the notorious long and short term deviations from strict periodicity present in Be stars make it difficult to rigorously demonstrate the connection between the VIR variability and the polarization variations, we have been able to find specific modes that are consistent with the observed V/R line profile variations together with the suspected polarization cycles.

Several bright O stars were monitored by broadband optical linear polarimetry at the University of Texas McDonald Observatory in conjunction with ultraviolet and optical spectroscopy during a series of international multiwavelength campaigns from 1986 through 1992. With a typical instrumental uncertainty of about 0.03% for a single observation, no polarization variability was detected at the 3σ level for any of the program stars. However, two of the stars observed during the 1991 October campaign have small-amplitude periodicities in polarization that are significant because they match those found by Kaper et al. (1997) in the simultaneous optical and ultraviolet spectra.

As part of our search for the origin of stellar-wind variability, we have conducted simultaneous ultraviolet and Hα spectroscopy of a number of bright O stars. The observed changes in the Hα line occur at low velocity (0 − 0.2v∞) on timescales that are characteristic of the development and evolution of discrete absorption components (DACs) in UV resonance lines. In some cases, a direct relationship is found between the changes occuring in the Hα line and subsequent variations in the high-velocity stellar wind. On the basis of this relationship, the appearance of a DAC in the UV resonance lines can be predicted from (ground-based) Hα observations. These observations show that the stellar wind is variable down to regions close to or at the stellar surface. Since the timescales of the variations can be related to the rotation periods of the stars in our sample, we propose that a stellar magnetic field (which remains undetected) might play an important role in affecting the base of the stellar wind. The observed variations are interpreted in terms of corotating wind structures, similar to the Corotating Interaction Region (CIR) model proposed by Mullan (1986) and recently simulated by Cranmer & Owocki (1996).

A detailed analysis of a very rich collection of spectroscopic and photometric observations of the bright Be star phi Per is presented. Earlier reports that phi Per is a double-lined spectroscopic binary consisting of two emission-line objects are confirmed. An orbital solution based on the emission-wing radial velocities for both stars has led to a determination of the orbital elements which defines the correct orbital phases. All data since the beginning of this century can be reconciled with a constant orbital period of 126(d).6731 +/- 0(d).0071. The new orbital solution gives lower masses than those found by earlier investigators, namely M(1) sin(3) i = 16.35 M(.) and M(2) sin(3) i = 1.69 M(.). These masses are in agreement with the recently derived spectral classes B0.5IVe and sd06: for the primary and secondary, respectively.Long-term light variations are positively correlated with the emission strength, and in the U-B vs. B-V diagram the object has changed its apparent photometric type from a B supergiant toward an MS object. Dereddening of the mean seasonal UBV magnitudes from recent years (when spectra show the weakest recorded Balmer emission) leads to a photometric spectral type a bit earlier than B1V. An important and exciting finding is that the emission lines of both stars have been weakening simultaneously in recent years, which seems to indicate some kind of interaction between the binary components.The presence of rapid light variability is confirmed, but its more detailed analysis is postponed for another study. Low amplitude orbital light variations with rather complicated light and color curves are found after the removal of long-term and rapid changes. The principal maxima and minima of the orbital light curves can be traced in both old and new photometry and can probably be attributed to specific circumstellar structures whose signatures are also seen spectroscopically.Finally, systematic secular changes in the shape and amplitude of the orbital radial-velocity curve of the Balmer shell lines are found, based on 1024 radial velocities spanning nearly a century.