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Astron. Astrophys. 322, 493-506 (1997)

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1. Introduction

The study of pulsating stars is of great importance for the construction of accurate stellar models, which provide in their turn valuable clues to our understanding of stellar evolution. It has indeed become clear that with detailed observational studies of the pulsations one can probe the internal structure of the stars, and hence confront the results with current theoretical stellar models (see e.g. Winget et al. 1991, Matthews 1993, Breger 1995, Dziembowski & Jerzykiewicz 1996).

The [FORMULA] Cephei stars are one of the best studied groups of early-type pulsating stars, in which both radial and non-radial, and mono- and multiperiodic pulsations occur. Only recently it was shown that the [FORMULA] -mechanism acting in a destabilising layer of the heavier elements is successful in explaining the observed [FORMULA] Cephei-type pulsations (Cox & Morgan 1990, Dziembowski & Pamyatnykh 1993). As the pulsation mechanism in these stars is still being tested observationally, the detection and identification of pulsation modes is of great importance for a confrontation with the recent theoretical pulsation models.

The bright pulsating star [FORMULA] Cep (HD205021, spectral type B2III, V=3.2, [FORMULA] 25 km/s) is the prototype of this class of [FORMULA] Cephei stars. The radial-velocity variations of this star were first detected by Frost (1902); the star is a radial pulsator with a period of 0.1905 days. Since then, line-profile variations have been found which are periodic at time scales of hours to decades.

Pigulski & Boratyn (1992) found that [FORMULA] Cep is the primary of a triple system; the secondary revolves around the primary with an orbital period of 91.6 [FORMULA] 3.7 year. The secondary has been resolved by speckle interferometry (Gezari et al. 1972), and the tertiary has a separation of 13.4 arcseconds (Heintz 1978).

A magnetic field strength of 810 [FORMULA] 170 G was measured by Rudy & Kemp (1978). Henrichs et al. (1993) and Veen et al. (1996) present measurements which emphasize the variable nature of the magnetic field strength of [FORMULA] Cep.

Besides the variable nature of the star itself, [FORMULA] Cep is also known to show variability of its stellar wind (Fishel & Sparks 1972). Studies of the ultraviolet (UV) wind lines of [FORMULA] Cep (Fishel & Sparks 1981) revealed a 6 or 12 day period. Henrichs et al. (1993) and Veen et al. (1996) identify the periodicity of the UV wind variability with the rotation period of a magnetic dipole, which implies a rotation period of the star of 12 days.

Long-term cycles of H [FORMULA] absorption and emission phases have been observed; the latest emission phase is reported by Mathias et al. (1991) and Kaper & Mathias (1995). Kaper et al. (1996) discuss whether the emission phases might be associated with the binary nature, with a sudden change of the amplitude of the radial pulsation, or with the magnetic activity of the star.

Aerts et al. (1994) discovered short-term multi-periodicity in the variations of optical lines of [FORMULA] Cep. They identified the frequency of the radial pulsation [FORMULA] =5.25 cycles/day, and found two more frequencies, [FORMULA] =5.38 c/d, [FORMULA] =4.92 c/d, close to the main frequency. The amplitude of these variations was found to be much smaller than that of the radial pulsation. Aerts et al. (1994) attributed these small variations at the newly found frequencies to non-radial pulsation modes of the star, but labelled the mode identifications as uncertain.

As a follow up on the work of Aerts et al. (1994), we investigate if any possible effect of the presence of a magnetic field can be found in the profile variations of the optical photospheric absorption lines of the star. Furthermore, we investigate the characteristics of the non-radial pulsation corresponding to the frequency [FORMULA]. Preliminary results of this work have been presented by Telting et al. (1995).

The plan of our paper is as follows. A brief description of the data is given in Sect. 2. In Sect. 3 we describe the period-search methods that we have used to analyse the data, and list all the frequencies that we find in the periodograms. The non-radial pulsation in [FORMULA] Cep is subject of the discussion in Sect. 4. We discuss the observed frequency splitting in Sect. 5, and give some concluding remarks in Sect. 6.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998