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Astron. Astrophys. 327, 145-154 (1997)

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2. The observations

Simultaneous five-channel observations of the brightness and polarization of RR Tau were carried out at CAO with the 1.25 m telescope using the Piirola (1975) UBVRI photometer-polarimeter. The observations were started in 1987-1988 by N.K.Minikhulov and were continued in 1992-1995 in cooperation with the Dodaira Observatory. Depending on the observational conditions the 10arc sec or 15arc sec diaphragms were used. At each observing night from 8 to 20 measurements of Stokes parameters were done. We used the star HDE 245817 = BD+ [FORMULA] 885 of Shakhovskaya et al. (1986) as the comparison star. The photometric data were reduced to Johnson's UBVRI photometric system. The accuracy of the photometric data is 0.m02 in [FORMULA] and in V and is 0.m01 in the other colour indices. At deep minima or at nights with the poor seeing conditions the accuracy is lower by about three times. The correction of the polarimetric measurements for instrumental polarization was made by the observations of polarimetric standards.

Table 1 3 presents the photometric and polarimetric observational data for RR Tau obtained at CAO. The total number of the observational nights is 117.

The observations at Dodaira were obtained from 1992 to 1994 at seven pass-bands (0.36, 0.41, 0.46, 0.53, 0.65, 0.70 and 0.76 [FORMULA] m) with a multi-channel polarimeter (Kikuchi 1988) mounted on the 0.91-m telescope. The observations were done with 15arc sec diaphragms. The photometric and polarimetric observations were always done simultaneously. The same comparison star as at CAO was used. The transformation of magnitudes and colours to the standard Johnson photometric system has been well established for normal stars using the data from the channels 1, 3, 4 and 6 for the UBV and R pass-bands, respectively.

The correction for instrumental polarization was done by the observations of nonpolarized stars listed by Serkowski (1973). The zero-point of the polarization angle was determined by assuming a polarization angle of [FORMULA] (at the V pass-band) for the highly polarized star 9 Geminorum. The efficiency correction was made by inserting a quartz polarizer in front of a rotating achromatic half-wave plate, which modulates the signals with 20 Hz, followed by a Wallaston prism.

The total number of observational nights is 36. The results of the Dodaira photo-polarimetric observations of RR Tau are listed in Table 2.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998