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Astron. Astrophys. 339, 822-830 (1998)

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1. Introduction

CU Virginis (HD 124224, HR 5313) is one of the best studied Magnetic Chemically Peculiar (MCP) stars belonging to the Si-group. It shows light, spectrum and magnetic field variations with one of the shortest periods among the CP stars. The first photometry of CU Vir was made by Hardie (1958) who confirmed the period of spectroscopic variations P=0[FORMULA]52067 previously reported by Deutsch (1952). All subsequent spectroscopic and photometric observations up until 1985 (see Table 2 for references) appeared to follow the same period. After 1985, the situation changed. First, Adelman et al. (1992) performed a period analysis of CU Vir UBV and uvby photometry from 1955 to 1989 and found a satisfactory fit of the data with a constant period of 0[FORMULA]5206800. However, the authors mentioned an existance of the small phase shifts between some data sets. Pyper (1994, 1997), on the basis of her photometric observations from 1987-1996, found an increasing phase shift of the light minima after 1985 (Fig. 1) . At the same time the shapes of the light curves remained essentially unchanged. Pyper found that the 1987-1996 observations did not fit the period of Adelman et al. (1992), but were better fit by a longer period of 0[FORMULA]5207030. A similar phase shift was also found by Kuschnig et al. (in prep.) in their spectroscopic study of CU Vir. All these facts forced us to make new spectroscopic and magnetic field observations of CU Vir and to repeat a careful analysis of all available data from 1955 to present.

[FIGURE] Fig. 1. Photometric U+u variations plotted with the period P1=0[FORMULA]5206778. Table 1, data set #1 are open squares; data sets #10-11 are closed triangles; and data set #26 are open circles. All data are normalized to correspond to Hardie's U magnitudes.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998