Astron. Astrophys. 355, 607-616 (2000)

1. Introduction

Photometric studies of Wolf-Rayet (WR) stars during the past decades (e.g. Moffat & Shara 1986; Lamontagne & Moffat 1987; van Genderen et al. 1987; Balona et al. 1989; Robert et al. 1989; Gosset et al. 1990; Antokhin et al. 1995; Marchenko et al. 1998a, b) have revealed light variations of several per cent (up to 0.1 mag) on time-scales (typically) of days. WR stars are generally believed to be evolved Population I stars, descendants of Of-type stars (Maeder 1996). They exhibit strong, dense winds (mass loss rates of to yr^-1) which, in most cases, hide the stellar surface. The wind-flow is dependent on time. Moffat et al. (1988, 1994) and Robert (1994) discovered the existence of small, outward moving wind condensations, which they called propagating blobs. Unlike most O-type stars, the continuum light of many WR stars originates from a layer in the dense wind (), a "pseudo-photosphere" (van Genderen et al. 1987), which could be inhomogeneous because of dynamical wind instabilities. The brightness variations of some WR stars proved to be periodic and are possibly due to binary or rotation effects. Core (photospheric) eclipses as well as atmospheric eclipses have been observed. The latter are characterized by only one V-shaped minimum on the light curve, which is caused by the atmospheric eclipse of an O-type star by the WR star's extended wind (Lamontagne et al. 1996). Random light variations are common in WR stars and they are often superimposed on the regular (binary) variations, increasing the "noise" and sometimes even totally disturbing the underlying regular light variations. Marchenko et al. (1998b) suggested that random light variations (light scatter) may be caused by short-lived, core-induced, multimode fluctuations, propagating in the wind. Other causes of variability, such as radial pulsations (Maeder 1985) non-radial pulsations (Vreux 1985; Antokhin et al. 1995; Rauw et al. 1996) and axial rotation (Matthews & Moffat 1994) have been proposed for WR stars. Occasional "eclipses" caused by dust formation in late-type WC stars have been studied by Veen et al. (1998).

WR 137 is a well known dust maker (Williams 1997; Marchenko et al. 1999). However little is known about long-term light variations for that star and its binary status is still uncertain. WR 140 is another repeating dust maker (Williams 1997). The orbit is well determined. Because of its high eccentricity () the strongest wind interaction occurs at periastron passage. During the last periastron passage in 1993, WR 140 received much attention and has been studied at different wavelengths from X-ray to radio. However, only a few photometric studies in the optical were carried out so far and the long-term behaviour of that star is not known. Both stars WR 137 and WR 140 are included in the infrared study by Williams et al. (1987a) and reported to have dust shells.

WR 148 is a good candidate for a WR + c (WR plus compact companion) binary. There is some controversy about the light variations concerning the period and the shape of the light curve. Marchenko et al. (1998a) were not able to detect the 4.31 d binary period in the HIPPARCOS photometry data, otherwise well known from ground-based observations (Marchenko et al. 1996). The very "noisy" light curve and unusually broad minimum need further investigation.

WR 153 is a quadruple system (Massey 1981), containing a WN + O and an O + O system, or two WN + O pairs (Panov & Seggewiss 1990). During the past years, several photometric studies have been carried out. Yet the light variability of the two pairs could not always be unambiguously separated (Lamontagne et al. 1996). Our aim is to try to solve some of these controversial questions.

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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