Astron. Astrophys. 355, 1015-1030 (2000)

1. Introduction

The satellite Hipparcos gave us an unbiased view of the occurence of stellar variability in our close environment and led to the discovery of 267 new variable B-stars. Waelkens et al. (1998) classified some 100 of them as new "Slowly Pulsating B-stars" (SPBs), a group of mid-B type variables pulsating in high-radial-order g-modes (Waelkens 1991). Since these modes penetrate deep into the stellar interior, this class of variables is very promising from an astroseismological point of view. All previously known SPBs are multiperiodic. Their pulsation periods range from 1 up to 3 days, resulting in beat periods of the order of months/years. Therefore, a long time-base is needed to disentangle the complete frequency spectrum and to perform mode identification.

In order to study their pulsational behaviour, we selected the brightest southern Hipparcos SPBs and started a long-term spectroscopic and photometric follow-up campaign in 1996. Also a few previously known SPBs are included in our target list. The selection of the targets is described in detail by Aerts et al. (1999), who list the stellar parameters in their Table 1. This sample is unbiased regarding spectral type, periodicity, binarity, and rotation velocity. Consequently, our targets are interesting objects to study possible interactions between these different properties.

Table 1. Overview of the different ground-based observation campaigns of our SPB-project.

In this first of a series of papers, we present (new) observational evidence that (at least) 8 of our targets are spectroscopic binaries with a pulsating component. For these multiple systems, we determine the radial velocity of the observed component(s) and derive an orbital solution. We use these orbits to shift the spectra to the velocity frame relative to the B-star itself. Afterwards, we search for evidence of at least one pulsation mode in the first normalised moment as defined by Aerts et al. (1992) and confront these results with those of a frequency analysis of the Geneva photometry and the Hipparcos photometry. Aerts et al. (1999) already searched for the main frequency in the single target SPBs but postponed to present the results for the binaries. This paper should be considered as the "twin"-paper for the binaries. Subsequent papers will be devoted firstly to a detailed frequency analysis of the spectra and the photometry and secondly to the interpretation of the observed variability in terms of non-radial pulsations for both the single and the binary SPBs in our sample.

The plan of this paper is as follows. The observations performed during the last two years, and the reduction of the data, are described in Sect. 2. Sect. 3 is devoted to the determination of the radial velocity. The searches for the orbital parameters and for the first pulsation frequency are described in Sect. 4. In Sect. 5, we give an overview of our results. Finally, a discussion of our results and some future plans are given in Sect. 6.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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