Periodic microvariation of B416, a new luminous blue variable in M33

We report optical photometric and spectroscopic observations of the M33 star B416, establishing it as the fifth identified luminous blue variable (LBV) in that galaxy. The B magnitude of the star varies with a period of 8.26 d and an amplitude of 0.03 mag. The red end of the optical spectrum does not seem to partake in these oscillations, which we identify as the microvariations, known to persist in LBV stars. B416 is the first LBV with such oscillations that are strictly periodic and coherent over at least 10 years. The brightness of the star is also varying on a longer (years) time-scale, again mainly in its blue light. The spectrum has most of the characteristics of an LBV in quiescence, showing Balmer, He I, Fe II and [Fe II] emission lines. The flux and profile of some of these lines are varying, but the scarcity of the observations prevents us from establishing their characteristic time-scale. B416 is at the centre of an H II region, another indication of the relatively low age of this stellar system.