Long-term photometry of the Wolf-Rayet stars WR 137, WR 140, WR 148, and WR 153

In 1991, a long term UBV-photometry campaign of four Wolf-Rayet stars was s tarted using the 60 cm telescope of the National Astronomical Observatory R ozhen, Bulgaria. Here we report on our observational results and discuss th e light variations. The star WR 137 was observed during 1991 - 1998. No indications of eclipses were found, though random light variations with small amplitudes exist, wh ich are probably due to dynamical wind instabilities. WR 140 was also monitored between 1991 and 1998. In 1993, a dip in the ligh t curve in all passbands was observed shortly after periastron passage, wit h amplitude of 0.03 mag in V. This is interpreted in terms of an "eclipse" by dust condensation in the WR-wind. The amplitude of the eclipse increases towards shorter wavelengths; thus, electron scattering alone is not suffic ient to explain the observations. An additional source of opacity is requir ed, possibly Rayleigh scattering. After the eclipse, the light in all passb ands gradually increased to reach the "pre-eclipse" level in 1998. The very broad shape of the light minimum suggests that a dust envelope was built u p around the WR-star at periastron passage by wind-wind interaction, and wa s gradually dispersed after 1993. Our observations of WR 148 (WR + c?) confirm the 4.3 d period; however, the y also show additional significant scatter. Another interesting finding is a long-term variation of the mean light (and, possibly, of the amplitude) o n a time scale of years. There is some indication of a 4 year cycle of that long-term variation. We discuss the implications for the binary model. Our photometry of WR 153 is consistent with the quadruple model of this sta r by showing that both orbital periods, 6.7 d (pair A) and 3.5 d (pair B), exist in the light variations. A search in the HIPPARCOS photometric data a lso reveals both periods, which is an independant confirmation. No other pe riods in the light variability of that star are found. The longer period li ght curve shows only one minimum, which might be due to an atmospheric ecli pse; the shorter period light curve shows two minima, indicating that both stars in pair B are eclipsing each other.