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Astron. Astrophys. 332, 857-866 (1998)

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4. Discussion and conclusions

We have investigated the photometric characteristics of four variable LMC supergiants ([FORMULA]  Cyg variables), three of which are strong emission-line objects and two are suspected LBV candidates. According to the photometric characteristics these two can be considered as true active LBVs near minimum light, but not spectacular.

4.1. R 99 = HDE 269445

The spectroscopic classification of this peculiar emission-line star met with difficulties due to its contradictory spectral characteristics. Classifications, based on spectra made between 1973 and 1994 and depending on the criterea used (Walborn 1977, 1982; Shore & Sanduleak 1984; Stahl et al. 1984; Bohannan & Walborn 1989; Pasquali et al. 1997a, b; Crowther & Smith 1997), run from OBf:pe to B0.5 Ia, WN9, and WN10h. In the optical region it has a pure emission-line spectrum (Walborn 1982; Bohannan & Walborn 1989). A luminous disk might be one of the reasons for the controversy (Stahl & Wolf 1987). Anyway, it is at least a member of the group of Ofpe/WN9 stars (Bohannan & Walborn 1989).

Whether the varying classification is partly due to the VLT-SD cycle (Sect. 3.1) with a time-scale of [FORMULA]  3 decades is questionable, because of its small range of [FORMULA].

From the photometric point of view R 99 is peculiar, too. The light and colour ranges of the [FORMULA]  Cyg-type variations are larger by a factor 4 and 5-10, respectively, (within a two-year interval, thus independent of the VLT-SD cycle) than those in other [FORMULA]  Cyg variables (see Fig. 13 in van Genderen et al. 1992 and Fig. 6 in van Genderen et al. 1990).

Further, the [FORMULA]  Cyg-type variations seem to be subject to at least two types of oscillations. The first one has a period of 2.088 d. With this period its colour curves are not all precisely in phase with the light curve. This is not a normal habit for [FORMULA]  Cyg variables. When the light ranges in the W passband are significantly smaller than those in the U passband, they appear to be modulated by the longer period of 9.98 d. Normally, light ranges in these stars increase to shorter wavelengths, but here the U passband is at the longer wavelength. These peculiarities could be due partly to emission-line variations originating in an extended envelope and/or luminous disk.

R 99 can be considered as an active LBV, but only subject to the longer type of SD phase: the VLT-SD cycle, with a small range of [FORMULA]. AG Car, S Dor and other LBVs show this type of SD phases with ranges up to [FORMULA] lasting a few decades. Besides, they are also subject to a second type of SD phase: the "normal SD phase", superimposed on the first one (van Genderen et al. 1997a, b). As concluded by different researchers, SD phases are mainly caused by a slow radius and temperature variation (Leitherer et al. 1989; de Koter et al. 1996; van Genderen et al. 1997a).

Other Ofpe/WN9 stars are still considered as dormant LBVs by Pasquali et al. (1997a, b; because no associated circumstellar nebula has been detected and resolved), Pasquali (1997) and Crowther & Smith (1997). In the latter paper the spectral characteristics of R 99 are compared with those of the WN10h star and supposedly dormant LBV HV 5495 (= HDE 269582 = BE 294) (Bohannan 1989). According to the photometric study of van Genderen & Sterken (1996), it is also an active LBV (although no associated circumstellar nebula has been detected), but near minimum brightness between 1989 and 1993, only showing normal SD phases, with amplitudes of [FORMULA] to [FORMULA] and a time scale of 1 to 2 y. Between 1890 and 1910 it showed a VLT-SD phase with a range of [FORMULA] and a cycle length of 20 y according to the photographic observations by Hoffleit (1940).

4.2. R 103 = HDE 269546

This supergiant was classified as B3 Ia by Feast et al. (1960) and as B5 Ia by Ardeberg et al. (1972). Considering its total light range, it has a hypergiant character but the precise quasi-period is uncertain; the time-scale of the light variations is 3 to 4 weeks.

4.3. R 123 = HD 37836

It is difficult to define a spectral type for R 123. It has clear P Cyg characteristics: Bpec (Feast et al. 1960), or a late O-type (Stahl & Wolf 1987). Like R 99 (Sect. 4.1), the optical spectrum is dominated by emission lines, presumably from a disk (Stahl & Wolf 1987).

Photometrically, R 123 is complicated as well and subject to at least four types of light variation (Sect 3.3). The first one has a time scale of a few decades to a century, since R 123 was [FORMULA]   [FORMULA] brighter in the second half of the 19th century. The second one has a cycle length of 292 d and a range of [FORMULA] with colours red in the maxima and blue in the minima. This points to an S Dor-variation. Thus, both types of variation likely represent the VLT-SD and normal SD phases, respectively. Incidentally, such a small range for the normal SD phases is not exceptional, since AG Car also showed them near minimum brightness. It is of interest also to note that in AG Car the (stable) period is 371 d (van Genderen et al. 1997a; Sterken et al. 1996), not much longer than the 292 d of R 123.

The two other types of variation are caused by [FORMULA]  Cyg-type variations with amplitudes of [FORMULA], and periods of 3.910 d and 1.342 d, dominant during the first two and last cycles of the 292 d variation, respectively. These periods are much shorter than for AG Car's [FORMULA]  Cyg-type variations near minimum light: [FORMULA]  2 weeks (van Genderen et al. 1990). Incidentally, AG Car's spectral type then was WN11 (Smith et al. 1994).

We conclude that R 123 is an active LBV, but much less spectacular than AG Car, S Dor, etc.

4.4. R 128 = HDE 269859

According to Walborn et al. (1991) R 128 has emission lines and shows an N-deficiency. It has been classified as B2 Ia. Our detailed photometry shows very complicated fluctuations on a varying time-scale of a week to a month with a total range of [FORMULA]   [FORMULA]. No reliable period or quasi-period could be found. Colours are generally blue at maximum light and red at minimum light, which is normal for [FORMULA]  Cyg variables.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998