BVRI Photometry of the CX Cephei System (WR 151)

We have obtained 699 new BVRI observations of the O5 + WN5 eclipsing binary system CX Cephei (WR 151), plus 126 more observations in V only. Our light curves are consistent with previous studies, showing a primary minimum (where the O5 star is eclipsed) of approximately 0.1 mag depth and a much smaller secondary minimum with an approximately 0.03 mag depth. Using the PHOEBE interface to the Wilson-Devinney computer code, we were able to obtain a reasonably satisfactory fit to these data, ignoring any possible contribution from atmospheric eclipse phenomena. The best-fit solution has i = 61.1° and results in masses of 36.8 M_☉ for the O5 star and 26.4 M_☉ for the Wolf-Rayet (WR) star. The binary system is detached. There is an asymmetry in the light curve, suggesting that the “leading side” of the O5 star (or the trailing side of the WR star) is brighter than vice versa. We also observed some features in the light curve that were persistent, but which we could not model.0 - C residuals relative to the PHOEBE fit reveal time variations with a total range of approximately 12% of the flux. Comparing our data with those of Lipunova & Cherpashchuk (1982), we find that the secondary minimum is less prominent today than it was in the 1980s. We were able to revise their period estimate to 2.12691 days