Photometric observations of YY orionis: New insight into the accretion process

We report on photometric UBVRI observations of the star YY Orionis unveiling the existence of a periodic component in its light curve, with period 7.58+/-0.15 days. We attribute this period to rotational modulation of the light curve by a large spot (or group of spots) located on the stellar surface. The amplitude variation as a function of wavelength indicates that the spotted area is hotter than the photosphere and covers about 10% of the projected stellar disk area, a larger surface than usually found for the hot spots of classical T Tauri stars. Comparing the properties of other spotted T Tauri stars with those of YY Ori, we find that the main difference lies in the much larger accretion rate of YY Ori. We show that in all cases the accretion luminosity emitted in the spotted regions represents only a fraction of the total accretion luminosity expected on the basis of the photospheric veiling and spectral energy distribution modelling. We discuss these findings and propose a new qualitative picture of the accretion process in YY Orionis and classical T Tauri stars. (C) 1996 American Astronomical Society.