Multi-colour photometric and spectroscopic monitoring of the WN5 star EZ Canis Majoris

We present and analyse photometric and spectroscopic observations of the WN5 star EZ Canis Majoris obtained over a period of 7 years. We discuss the changing light curve, the shift in phase of the maxima and point to flare type variability seen in one night. Small amplitude variations are reported in another night. We have investigated the change of the average visual magnitude over a time span of 18 years and found a tentative cyclic variation with a time scale of 2425d (6.6 yr) with a range of $\sim 0\hbox{$.\!\!^{\rm m}$}07$. This, of course, should be verified. If true, a precession phenomenon may offer an explanation. The trend of the maximum light amplitude of the $3\hbox{$.\!\!^{\rm d}$}766$ cycle is also investigated and it shows a saw-tooth character with a timescale of $\sim 400^{\rm d}$. A possible relation with the magnetic activity of the star is discussed. We conclude that the line emission variability can be caused by both a single star model with an ever-changing wind and a binary (WN+NS) model