Wra 751, a luminous blue variable developing an S Doradus cycle

Aims. The object Wra 751 is a luminous blue variable that lately exhibits strong changes in light and color. We summarise the available photometry of Wra 751, present new photometric observations, and discuss these data with special attention on the systematic differences between the various data sources. In addition, we establish an empirical relationship between $b-y$ and $B-V$ for this class of stars. Methods. Wra 751 is a strong-active member of the S Dor class, and it exhibits very-long term S Doradus phases with an amplitude of about two magnitudes in V and a cycle length of several decades. The associated $B-V$ colour-index amplitude is about 0.4 mag. At this moment this LBV, which is the reddest member of the class, is going through the bright (and red) stage of a long-term S Dor cycle. Results. The S Dor behaviour of this system shows some resemblence to the temporal characteristics of the Galactic LBV AG Car, because the time scales and amplitudes of light and colour variability are very similar