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Astron. Astrophys. 331, 171-178 (1998)

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2. Observations

The observations were carried out on eight nights during January and March, 1995 using the 90 cm telescope at Sierra Nevada Observatory, Spain. The photometer attached to this telescope is a six-channel uvby [FORMULA] spectrograph photometer for simultaneous measurements in uvby or in the narrow and wide H[FORMULA] channels, respectively, using uncooled EMI photomultipliers type 9789 QA (Nielsen, 1983).

For these observations, HD 115271 (V=5.m 79, A7V) was used as the main comparison star with HD 116127 (V=6.m 6, F2) and HD 116303 (V=6.m 35, A7) as check stars. The sequence was, generally, C1,C2,star,C1,star. Sky measurements were made every 2 or 3 cycles. C3 was only observed during the two first nights in order to prove the constancy of the comparison stars. 831 uvby measurements were collected for the variable, more than 600 for each of C1 and C2 and more than 100 for C3. In addition, some additional data were also obtained in H[FORMULA] for each of the four objects. Each integration consisted of 30 s. This gives us, for any of the stars, an internal error in each observation better than 0.m 0012 in the u filter and C2, namely the worst case. This precision is very important for the calibration of the physical parameters and, especially, for the frequency analysis of the variable and to analyse the phase shifts and amplitude ratios between different colours. Also, in the H[FORMULA] system, each observation consisted of 30 s which yields an internal error better than 0.m 0017 in the n band and C2, being the worst case.

During the observations reported here, neither of the comparison stars showed any sign of variability. On every night, the standard deviations for C2-C1 differences were always better than 0.m 006, 0.m 003, 0.m 003 and 0.m 003 for u, v, b and y, respectively. This demonstrates the excellent quality of the sky during this campaign. In addition, the stability of the instrumental system was very good. In fact, the mean values obtained for differences C2-C1 on the eight nights were always the same within 0.m 001, as standard deviation, for any of the filters and colour indices.

To transform our data into the standard system we have used the same procedure described in Rodríguez et al. (1997). The data obtained, as magnitude differences variable minus C1=HD 115271 in the standard system versus Heliocentric Julian Day, have been deposited in the Commission 27 IAU Archives of Unpublished Observations, file 332E, and can also be requested from the authors. As an example, the v observed light curves are plotted in Fig. 1 with the Fourier fitting obtained in Sect. 3.1.

[FIGURE] Fig. 1. Observed light curves of 20 CVn in the v band with the Fourier fitting versus Heliocentric Julian Day

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© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998