UBV (RI)c photometric monitoring of Orion population stars in the southern hemisphere.

UBV(RI)C photometry on a sample of southern T Tauri stars, selected from the Herbig and Bell (1988) catalog, is reported and four new candidates (AS 205, Wa Oph/2, Wa Oph/3, and Wa CrA/2) for rotational modulation are singled out from the sample. The color trend for the stars showing variability is considered. Typically, colors become redder when the star is fainter, complying with the general behavior observed in T Tauri stars. This can be interpreted as due to the (variable) presence of surface inhomogeneities (either dark or bright) or, in some cases, it could also be consistent with the hypothesis of variable circumstellar extinction. Estimates of gross spot properties (e.g., temperature and filling factor) are derived for the four stars showing possible periodicity, from the amplitudes of the variations observed at different wavelengths, by use of a simple blackbody model.