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Astron. Astrophys. 333, 565-570 (1998)

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1. Introduction

Luminous Blue Variables (LBVs) are massive early-type stars exhibiting spectroscopic and photometric variability with different time-scales. Their photometric variability is, generally, described as semi-regular or semi-periodic. However, recent evidence (Sterken et al. 1997, van Genderen et al. 1997a,b) suggests that this photometric variability may be described by the combined effect of multi-periodic oscillations and some degree of stochastic variability. In particular, Sterken et al. (1996) demonstrate the existence of a stable pulsation period of 58 days in the case of [FORMULA]  Car, while van Genderen et al. (1997a,b) describe the existence in most LBVs of two kinds of S Dor phases, viz. normal (SD) phases and, as they define, very-long-term (VLT-SD) phases. Both kinds are of a recurrent nature-that is, their appearance is not periodic, but cyclic. The evidence for the existence of both kinds of oscillations comes from an analysis of several decades (to more than one century) of photometric observations and magnitude estimates. The study of these oscillations in LBVs is an important tool for understanding their internal and/or atmospheric structure and the role they play in the episodic mass-loss events displayed by these objects.

The LBV character of R 40 (HD 6884, [FORMULA], [FORMULA]) was discovered by Szeifert et al. (1993) as the star had become brighter in the visual range by about 0:m5 between 1986 and 1993; the brightening was accompanied by a change in spectral type from B8Ie in the late 1950s to A3Ia-O in 1993, the true signature of an LBV turning cooler when becoming brighter while going through a (mild) active phase (or S Dor phase in the nomenclature conceived by van Genderen et al. 1997a). Szeifert et al. (1993) discerned a quasi-period of about 120 days, and assigned the fundamental stellar parameters [FORMULA] [FORMULA] K, [FORMULA] [FORMULA], [FORMULA] [FORMULA], [FORMULA] [FORMULA] and [FORMULA] [FORMULA].

R 40 is a touchstone in two respects. First of all, its brightening during the last decade allows the study of the microvariations of an LBV in a stage intermediate between quiescence (hot early-type [pre-]LBV) and maximum state (cool star surrounded by a slowly-expanding envelope implying spectral type A) while roughly maintaining a constant bolometric magnitude. In addition, the study of such an object may lead to an answer to the question whether the microvariations during maximum light are of a different nature from those seen in quiescence (see the analyses by van Genderen et al. 1997a,b).

In this paper we present a study of the light variations of R 40 involving all available photometric data originating from photometric monitoring during the last 15 years, including data that were not available at the time Szeifert et al. (1993) announced their discovery that R 40 is an LBV. This particular case of R 40, characterised by a steady (quasi-linear) behaviour during the phase of light increase, enables us to test the possible objection that the derivation of the underlying pattern of variability of the SD phenomenon is subjective (since it relies on eyeball inspection of and filling in of gaps in complex light curves) thwarted by far more intuition and wishful thinking than should be allowed in any search for periodicities in a not-continuously observed natural phenomenon. Indeed, the steady pace of the star's brightness increase allows us to derive the underlying cyclicities by two independent approaches.

Throughout this paper we discuss differential photometry, in the sense that the variability of R 40 is discussed in terms of the differential magnitude of R 40 relative to the corresponding signal for a comparison star in each Strömgren band, and in the Walraven and Johnson V bands.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998