1998A&A...333..565S

Stroemgren differential photometry of R40 collected during the time interval 1986-1996 is analysed together with Walraven photometry. The gradual brightening of the star over the last 10 years can be described by a linear trend with superimposed oscillations (in v, b and y) with frequency 0.0008c/d (∼1300d cycle). We interpret these oscillations as ``normal S Dor'' phases, and suggest that the quasi-linear brightening of the star is the ascending branch of a growing very-long-term S Dor phase (VLT-SD), as found by van Genderen et al. (1997A&A...318...81V) in AG Car and S Dor itself. As R40 is now becoming fainter and bluer, the length of the VLT-SD cycle is about 20 years.