Astron. Astrophys. 355, 607-616 (2000)

2. Observations

In 1991 we started a long-term photometric study of the four WR stars at the National Astronomical Observatory Rozhen, Bulgaria, using the 60 cm telescope and the UBV single channel, photon counting photometer. The photometric equipment has been used for many years and proved to be very stable (cf. Panov et al. 1982).

Table 1 contains the comparison and the check stars used. Generally, a 20" diaphragm and an integration of 10 s were used. Each measurement consists of four consecutive integration cycles. An observing cycle was arranged in the following way: Sky - Comp - WR - Comp - Sky and was repeated 3 to 5 times, depending on the quality of the night. A separate measurement of the comparison star against the check star in the same way was obtained before or after the WR star observation. Thus a nightly mean was calculated from the 3 to 5 individual measurements. The standard error of the nightly mean is 0.003 - 0.005 mag in most cases. Reduction of the data was made taking into account dead-time effects, atmospheric extinction, and transformation into the standard UBV system. In the following tables we present the magnitudes in the standard system; for WR 137, WR 140, and WR 148 the data are magnitude differences in the sense: comparison star minus WR star. The contribution and of emission lines to the respective colours are taken from Pyper (1966) and included in Table 1 (last two columns). No corrections have been applied for emission lines in our data. However, it does seem possible to distinguish the continuum light from the emission line variations by comparing the light in the UBV passbands.

Table 1. Summary of data for the program stars. The last two columns contain the emission line-contribution to the colours (Pyper 1966)

Many WR stars show subtle short and long term variations as can be seen in the case of WR 148 (Sect. 3.3). Therefore it is preferable to use the same photometric equipment for long term studies. This reduces possible systematic effects caused by slightly different passbands or response curves.

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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