2005A&A...437..127L

We present the results from observing the circumstellar maser emission of the M-type supergiant S Per in the 6₁₆-5₂₃ water-vapour line at 1.35cm. The observations were carried out in 1981-2002 (JD=2444900-2452480) on the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory, Astrospace Center of the Lebedev Institute of Physics, Russian Academy of Sciences. The H₂O spectra obtained represent an unprecedented long, uniform dataset on this star. We discuss the properties of the optical and maser variations of S Per, together with particulars of the available VLBI maps. The close relation between maser and optical variations favors a model in which mass-loss is episodic. Changes observed in the total H₂O line flux follow the visual light curve with a delay of 0.01 to 0.5P, where P≃800 d is the mean light cycle for S Per. The feature at V_LSR=-44km/s flared in July 1988, which seemed to be the response of the maser to an unusually bright optical maximum. The position of the -44km/s feature on the VLBI maps coincides with the direction toward the optical stellar disc, which can be explained by amplification of enhanced stellar continuum by the H₂O line.