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Astron. Astrophys. 350, 566-570 (1999)

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1. Introduction

OT Gem (HD 58050, HR 2817, BD +15o1564, MWC 176, B2Ve, V=6.[FORMULA] 4-6.[FORMULA] 0, v sin i = 130 km s-1) is a bright Be star which exhibited strong spectral variations during the past 60 years. The first evidence of the emission changes was presented by Merrill & Burwell (1943). The spectroscopic behaviour between 1954 to 1975 is described by Hubert-Delplace & Hubert (1979), where the strong emission of Balmer and Fe II lines is clearly seen. The strength of the emission is steadily decreasing after the maximum in 1961-62, with intermittent variations. Hubert-Delplace et al. (1982) reported the H[FORMULA] line variation and showed that the strength of the emission was secularly fading from 1961, reaching a minimum at the end of 1980. The history of photometric and especially photoelectric observations of OT Gem is not too long. That OT Gem could be a rapid variable was first recognised by Hoffmeister (1934) who, using photographic photometry, detected the rapid light variations ranging from 6.[FORMULA] 0 to 6.[FORMULA] 3. Until the eighties, only two measurements in the UBV photometric system were obtained: one by Mendoza (1958) ([FORMULA], [FORMULA], [FORMULA]) and one by Crawford et al. (1971) ([FORMULA], [FORMULA], [FORMULA]). Based on visual photometry, Figer (1981) announced two types of variability of OT Gem, a 0.[FORMULA] 4 amplitude long-term increase of light from the end of 1980 until the beginning of 1981, and a periodic rapid variation with the strict period of 0.[FORMULA] 12500 (or 0.[FORMULA] 14286) with an amplitude of 0.[FORMULA] 15. Differential UBV photoelectric measurements made at Hvar Observatory during five nights in January 1982 by Bozic et al. (1982) did not show the presence of rapid variability. The lack of short periodic light variations was also independently confirmed by differential UBV measurements from Merate Observatory by Poretti (1982), who observed the star during three nights in January and March 1982.

Shortly thereafter, Berthold (1983) published a photographic light curve of OT Gem for the period from 1960 to 1980 based on the 272 Sonneberg Sky Patrol plates. In the first part of this period (JD 2437200-2440000) the light changes were irregular, while later some type of slow cyclic variations appeared. These variations do not exceed 0.4 of photographic magnitude. The last part of this light curve confirmed the light increase observed by Figer at the end of 1980. Alvarez & Schuster (1981) and Schuster & Guichard (1984) classified OT Gem as a possible variable from their 13-colour photometry at San Pedro Mártir. Their subsequent observations between 1980 and 1983 revealed large brightness and colour variations - see Schuster & Guichard (1984). In the summary report on UBV photometry of Be stars carried out at the Hvar Observatory between 1972 and 1990 by Pavlovski et al. (1997) and Harmanec et al. (1997), a small secular brightness decrease of OT Gem over four consecutive seasons was found. Ferro et al. (1998) analysed a set of photoelectric and visual measurements obtained in 1995-96 at La Luz Observatory and announced a light increase of OT Gem in October 1995 followed by a very active phase in which cyclic light variations appeared.

Photometric variations of Be stars usually occur on at least three time scales: long-term, medium-term and short-term ones - see, e.g., Harmanec (1983). Pavlovski et al. (1997) recognized a fourth possible class of photometric variability: a sudden light change. In their analysis of the photometric behaviour of the large set of Be stars based the Hipparcos photometry, Hubert & Floquet (1998) reported also several different types of medium-term changes: outbursts, fadings and quasi-periodic oscillations.

Our intention was to analyze the photometric behaviour of OT Gem over the whole period covered by photoelectric observations and to compare brightness and colour variations with the parallel spectral changes of the object.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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