Long uninterrupted photometric observations of the Wolf-Rayet star EZ CMa by the Toronto BRITE satellite reveal a very fast apsidal motion

Context. The variability of the Wolf-Rayet star EZ CMa has been documented for close to half a century, and a clear periodicity of similar to 3.7 days is established. However, all attempts to prove that it is a binary have failed because the photometric, spectroscopic, and polarimetric variations are not coherent over more than a few orbital cycles. Aims. In this Letter we show that the lack of coherence in the variability can be explained with a very rapid apsidal motion in a binary orbit. Methods. We measured the times of minima in a recently published exceptionally long photometric light curve obtained by the Toronto BRITE satellite. The apsidal motion and the system eccentricity are determined from the length of the time intervals between these minima, which alternate in their duration, following a pattern that is clearly associated with apsidal motion. These minima are superposed on brightness enhancements of the emission from a shock zone, which occur at about the times of periastron phases. Results. We determine the orbital periodicity, P-a = 3.63 d, and the period of the apsidal motion, U similar or equal to 100 d, which together yield an average sidereal period of P-s = 3.77 d. The eccentricity is found to be close to 0.1. The rate of periapsis retreat changes significantly over the period of observation and is determined to be -16 degrees P-a(-)1 at the beginning of the observing period and -10 degrees P-a(-)1 at the end. Conclusions. We demonstrate that by introducing a fast apsidal motion, the basic photometric variability is very well explained. The binary nature of EZ CMa is now established. This might imply that other apparently single Wolf-Rayet stars that emit hard X-rays, similar to EZ CMa, are also binaries.