2. The observations

Part of the data were taken in March 1990 and January 1992 at The Calar Alto Observatory (Almeria, Spain). The telescope used was the 1.5 m reflector equipped with either a multipurpose one-channel photoelectric photometer or, alternately, with a four-channel uvby-H spectrograph photometer, both using narrow and wide H filters. The last two photometric campaigns were carried out in April 1993 and May 1994 at The European Southern Observatory (La Silla, Chile) using the 0.5 m Danish telescope equipped with the same uvby-H four-channel photometer, with Strömgren uvby filters as well as Crawford's narrow and wide H filters.

The 1585 measurements of BH Vir were obtained in the Strömgren uvby system during four observing campaigns.

• four nights in March 1990 using the mono-channel photometer UBVuvbyH mounted on the 1.5 m telescope of the Observatorio Astronómico Nacional at The Calar Alto Observatory at Almeria, Spain,
• six nights in January 1992 using a four-channel photometer uvbyH mounted on the 1.5 m telescope of the Observatorio Astronómico Nacional at The Calar Alto Observatory at Almeria, Spain,
• eleven nights in April 1993 using a four-channel photometer identical to the one used in the previous campaign mounted on the 0.5 m Danish telescope at the European Southern Observatory (ESO), La Silla, Chile
• eight nights in May 1994 using the same four-channel photometer on the 0.5 m Danish telescope at the ESO, La Silla, Chile.

Details about the instrumental configuration and the reduction of the photometry can be found in Clement et al. (1997) and references included there.

HD 121935, (V=9.67, F4V) and HD 139643 (V=9.44, G0V), were used as comparison and check stars. Their magnitudes and spectral type being similar to BH Vir itself. Average magnitudes and colour indices for the comparison stars are given in the last two columns of Table 1 (click here), with an indication of their accuracy measured through the RMS dispersion of the observed values. In Table 1 (click here) is also given averaged magnitudes and colour indices for BH Vir in the eclipses and first quadrature. The constancy of the comparison star was checked every night. The internal RMS errors for the 569 differences (comparison- check) 0.012, 0.010, 0.015, 0.021, 0.017 in magnitudes for V, (b-y), , and respectively is of the same order as that obtained for main-program stars. Extinction corrections were applied based on nightly coefficients. The differential uvby light curves of BH Vir with respect to HD 121935 were calculated. Besides the program stars, a selected set of standard stars from the lists of Perry et al. (1987) and Olsen (1991), was measured during the best photometric nights in order to transform the instrumental magnitudes into the standard system, as the accuracy of our standard photometry is quoted in a few millimagnitudes (0.009 mag in the y filter).

The 1585 differential magnitude (BH Vir - HD 121935) values in the standard system are given in Table 2. Figure 1 (click here) presents the BH Vir differential light curve in the y filter in which the effect of activity is clearly visible.

Acknowledgements

The 1.5 m telescope at Calar Alto is operated by the Observatorio Astronomico Nacional. The 0.5 m telescope is operated by the Copenhagen Group. We want to express our gratitude to the observatories staff. This work has been supported by the Spanish Comision Interministerial de Ciencia y Tecnologia (PB90-0001-C02)