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Astron. Astrophys. 346, 134-138 (1999)

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2. Observations

HR 1960 has been measured in the Geneva photometric system (Golay, 1980) with the photoelectric photometer P7 (Burnet & Rufener, 1979) installed on the 40 cm and 70 cm Swiss Telescopes in La Silla (ESO, Chile). The photometric reduction procedure was described by Rufener (1964, 1985); the photometric data in the Geneva system are collected in the General Catalogue (Rufener, 1988) and its up-to-date database (Burki, 1998). An amount of 676 measurements has been obtained during the 16 years of the survey. For the present study, only 615 of these measurements have been used, those of quality weight 0 or 1 having been excluded (see Rufener (1988) for the definition of the weight). The major part of these data, i.e. 567 measurements, have been obtained between February 1987 and August 1989, when HR 1960 was used as a comparison star for the monitoring of SN 1987A (see Burki et al., 1989, 1991), together with HR 1744 (HD 24649). It is useful to note that: i) the results obtained on SN 1987A have not been affected by the variability of HR 1960, as the amplitude detected is very small; ii) the constancy of HR 1744 during our monitoring supplies the proof that the variability of HR 1960 (very small amplitude and period near one year) is not a spurious effect of the photometric reduction procedure.

In addition to these photometric measurements of HR 1960 in the Geneva system, 182 photometric measurements have been obtained by the Hipparcos satellite (ESA, 1997) in the range 7 891-9 052 (in HJD-2 440 000). Among these data, 101 are of very good quality (flag quality 0) and have been used in this study. Three values, which are at more than 3[FORMULA] from the fitted curve (see Sect. 4), have been excluded. This has allowed to confirm the characteristics of the variability found in Geneva photometry. To compare the magnitude Hp from Hipparcos with V, the relation between [FORMULA] and the Geneva colour index [FORMULA] has been used (see Carrier et al., 1999). Taking the mean value [FORMULA], it follows that for HR 1960 [FORMULA]. Then, the Hipparcos photometric data have been transformed into the V magnitude system.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999