A study of bright southern slowly pulsating B stars I. Determination of the orbital parameters and of the main frequency of the spectroscopic binaries

In 1996, we started a long-term spectroscopic and photometric study of 17 southern Slowly Pulsating B-stars. In this paper, we report our finding that at least 8 of them turn out to be spectroscopic binaries. We present the results of the determination of the orbits from the spectroscopic data. There is a great variety in the derived orbits. HD123515 and HD140873 were known as single-lined spectroscopic binaries, but both turn out to be double-lined. All the others binaries are single-lined. For HD140873 and HD177863, we find orbits with a large eccentricity of respectively e = 0.731 +/- 0.006 and e = 0.603 +/- 0.007. HD69144, HD92287 and HD169978 are three circular binaries with a very short orbital period (a few days). Since their photometric measurements are dominated by a (close to) sinusoidal variation with twice the orbital frequency, these stars are ellipsoidal variables. Their orbital periods are of the same order of magnitude as the periods of pulsation. After removing the orbit, we find the same first frequency in the residual radial velocities as in the gathered photometric measurements for 6 stars. For HD69144 and HD169978 we did not yet succeed in deriving an intrinsic period, although HD69144 has prominent line profile variations. HD169978 was misclassified as an SPB.