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Astron. Astrophys. 335, 995-1002 (1998)

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1. Introduction

Spectroscopic variability in the optical and ultraviolet wavebands is a widely observed characteristic of early-type stars. However, in most cases, the underlying mechanisms are still rather ill-defined. Only an extensive spectroscopic investigation allows to derive constraints on the time scales and the properties of these variations. The typical time scales range from a few hours to several days (e.g. Fullerton et al. 1996, Kaper et al. 1996, 1997). One of the most controversial issues is the connection between the variability at the photospheric level and in the stellar wind. In some stars, the line profile variability of photospheric absorption lines is attributed to pulsations (e.g. Kambe et al. 1997), whereas Kaper et al. (1997) suggest that a stellar magnetic field may be an essential ingredient for controlling wind variability.

In the present paper, we focus our attention on HD 192639, a rather poorly studied member of the Cyg OB1 association. On the basis of eye inspection of photographic spectra, Walborn (1972) assigned a spectral type O7 Ib (f) to this star; the tag (f) in this classification scheme indicates that the N iii [FORMULA] 4634-41 lines are clearly seen in emission whereas the He ii [FORMULA] 4686 line is seen neither clearly in emission nor in absorption. Using quantitative criteria, namely the ratio of the equivalent widths of selected lines, Conti & Alschuler (1971) assigned a spectral type O7.5 III f to HD 192639. More recently, using the same quantitative criterion on her spectra, Underhill (1995a) derived an O8 spectral type instead of O7 or O7.5. On the basis of the Stark broadening of the He ii lines, Underhill (1995b) further assigned a supergiant luminosity class to HD 192639, leading to a O8 I f classification.

In the HR-diagram of the Cyg OB1 association HD 192639 lies at a position typical for a blue straggler, i.e. beyond the turnoff point of the association and in the vicinity of the continuation of the ZAMS (Mathys 1987). The spectrum of HD 192639 shows some indication for the presence of CNO processed material at the surface of the star. In fact, Schild & Berthet (1986) found that the C/N abundance ratio is smaller in HD 192639 than in HD 193514, another member of the Cyg OB1 association with a similar spectral type. Herrero et al. (1992) also derived a slightly enhanced helium abundance in the spectrum of HD 192639.

Variability of the emission lines in the spectrum of HD 192639 has already been reported on various occasions (Mannino & Humblet 1955, Underhill 1995a). Underhill (1995a) noticed line profile variability over a few days in the H[FORMULA] and He ii [FORMULA] 4686 emission lines. Fullerton et al. (1996) discovered the existence of line profile variability in the C iv [FORMULA] 5801, 5812 absorption lines and in the He i [FORMULA] 5876 P-Cygni profile over an interval of about seven days. Finally, in the near infrared, Andrillat & Vreux (1979) reported the variability of the He i [FORMULA] 10830 emission line, also on a time scale of a few days.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998