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Astron. Astrophys. 326, 143-154 (1997)

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1. Introduction

In spite of considerable advance achieved in the past decade on the formation and evolution of massive stars, the evolutionary sequence, between O and Wolf-Rayet (W-R) stars, is still far from being established. Of prime importance for this topic is the investigation of a very small family of objects, the so-called Ofpe/WN9 transition stars, which seems to hold the keys for better understanding the physical characteristics of several massive star subclasses populating the upper part of the HR diagram. Ofpe/WN9 star s show the spectral features of both emission-line O star s and the later W-R types of the nitrogen sequence, i.e. a combination of high- (He II and N iii) and low- (He i and N ii) excitation emission features. The designation Ofpe/WN9 underlines the difficulty in distinguishing between these two subclasses. In fact, these star s were given either OIafpe or WN9-10 classifications by Walborn (1982 ) but were later revised to Ofpe/WN9 by Bohannan & Walborn (1989 ). Recently, three of the Magellanic Cloud "slash" star s (R 84, BE 381, and HDE 269927c) have been reclassified as WN9 by Crowther et al. (1995 ).

Anyhow, one of the most important aspects of these upper HR diagram objects is their close link with the Luminous Blue Variable (LBV) phenomenon (Stahl et al. 1983 ) which, according to current massive star evolutionary models (Maeder 1989 , Langer et al. 1994) represents a short stage in the evolution of O star s initially more massive than 60 [FORMULA] before the advent of the Wolf-Rayet phase. In fact Ofpe/WN9 star s have been described as quiescent LBVs (Bohannan & Walborn 1989 , Crowther et al. 1995 ).

At present, only 10 massive transition stars are known in the LMC (Bohannan & Walborn 1989 ). This small group should be investigated from every angle in order to gain insight into their status. For example, the stellar environment of these stars and their possible belonging to a massive star cluster deserves consideration. This new approach to the study of the so-called Ofpe/WN9 star s will be applied in this paper to R 84 (Feast et al. 1960 ). On the basis of sub-arcsecond images in the U, B, V, R and the near infrared H and K bands, we will try to resolve R 84 and its neighborhood. Furthermore, we will, for the first time, give accurate photometry of the unknown resolved components. R 84 is of particular interest since it is the only star of this class showing the signature of a late-type companion in its red and infrared spectroscopy and photometry (Allen & Glass 1976 , Wolf et al. 1987 , McGregor et al. 1988 ). Presently, we do not know whether the red companion, classified by Cowley & Hutchings (1978 ) as an M2 supergiant, is physically related to the Ofpe/WN9 or their association is just a line-of-sight effect.

R 84 has several other designations, mainly: H II 7 (Henry Draper (Extension) catalog), Brey 18 (Breysacher 1981 ), Sk- [FORMULA] 79 (Sanduleak 1970 ), S 91 (Henize 1956 ), WS 12 (Smith 1968 ). It lies in the central part of the OB association LH 39 (Lucke & Hodge 1970 , star #12) towards the southern edge of the bar. In this direction is also situated the LMC weak, filamentary H [FORMULA] nebulosity DEM 110 (Davies et al. 1976 ). Feast et al. (1960 ) classified it as Pec(uliar). Later, it got other classifications, as follows: WN8 (Smith 1968 ), OIafpe (Walborn 1977 ), WN9-10 (Walborn 1982 ), Ofpe/WN9 (Bohannan & Walborn 1989 ), WN9 (Crowther et al. 1995 ). The star has been found to show significant brightness and color variations (Stahl et al. 1984 ) as well as moderate spectroscopic variability in H [FORMULA] and He i [FORMULA] 4471 (Stahl et al. 1985 ). A detailed spectral analysis of R 84 was presented by Schmutz et al. (1991 ) using the observations of Stahl et al. (1985 ). On the basis of a non-LTE model for a spherically expanding atmosphere, which was the first of the kind for an Ofpe/WN9 star, they derived the stellar parameters of R 84. Hydrogen was found to be very depleted and it was found that R 84 had lost about half of its initial mass, and was probably a post-red supergiant. Recently Crowther et al. (1995 ), using an independent model but the same set of data, have confirmed Schmutz et al.'s (1991 ) findings.

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Online publication: April 20, 1998