New clues on the nature of the recurrent nova T Coronae Borealis from a light curve of the system spanning 40 years

We have analysed three large records of visual estimates of the magnitude of the recurrent nova T Coronae Borealis, spanning a time interval of nearly 40 years, from 1956 to 1995. We find in the light curve the well-known 227-d periodicity of the binary cycle. The brightness of the system declines with an average rate of ∼10^-5 mag d^-1. An oscillation, possibly periodic with a period of 9840 d, with an amplitude of 0.09 mag, is superposed on this linear fading. The brightness 'noise' level of the system is also modulated with this periodicity. The origin of the linear fading, as well as of the 9840-d brightness oscillations, is probably in or around the hot component, while most of the short term 'noise' fluctuations originate in the photosphere of the red giant star of the system. The 'noisy' regions seem not to be homogeneously distributed over the surface of the star. Comparison of the binary light curve with published radial velocity data indicates that the spin period of the giant is longer than the orbital period of the system. The visual luminosity of the hot component is between 0.09 and 0.4 of the luminosity of the red giant.