1 Introduction

One of the topical problems in the current research of early-type stars is to understand the nature of rapid line profile changes observed for many hot stars. Speculations about the crucial role of duplicity for either triggering or at least modifying stellar pulsations are not new. Fitch (1967, 1969) called attention to the fact that many known $\beta$ Cep stars are spectroscopic binaries and demonstrated, on particular examples, that the tide-raising potential does indeed affect the pulsations. Osaki (1971) pointed out that slow periodic changes of the mean ($\gamma$) value of the pulsational ($\beta$ Cep) RV curve need not be due to orbital motion in a binary system but may also be caused by a superposition of two oscillations with similar periods. He suggested this as an explanation for Fitch's (1969) result for $\beta$ Cep. Pigulski & Boratyn (1992) demonstrated, however, that the secular changes in the main pulsational period of $\beta$ Cep are caused by light time effect as the star moves in orbit with a distant binary companion, discovered by speckle interferometry. On the other hand, an archetype line profile variable $\varepsilon$ Per (HD 24760) was found to be a 14.1-d spectroscopic binary in an eccentric orbit, with a possible distant tertiary - see Harmanec (1989), Harmanec & Tarasov (1990), Tarasov et al. (1995) and De Cat et al. (2000). Kato (1974) investigated the conditions under which non-radial oscillations could either be excited or become multi-periodic due to a resonant interaction with the tidal mode in binary systems. One of the principle problems in investigating pulsations in binaries is the disentangling of the variations due to orbital motion and of the short-term oscillations. This disentangling was successfully applied e.g. to the $\beta$ Cep stars $\beta$ Sco A (Holmgren et al. 1997) and ARCas (Holmgren et al. 1999).