Periodic light variations in four pre-main-sequence K stars

Four pre-main-sequence K stars with weak line emission (HD 283447, V410 Tau, and two of the X-ray stars studied by Feigelson and Kriss, 1981) have been monitored with UBVRI photometry during seven consecutive nights in 1981 October. All four stars show quasi-sinusoidal light variations, apparently due to the presence of large starspots. The light variations have amplitudes of from 0.06 to 0.23 mag in yellow light and periods ranging from 1.9 to 4.1 days. Several significant UV flares were also observed. The observations indicate that these young stars possess magnetic surface activity similar to that seen in the RS CVn and BY Dra stars. The observed stars fall within rather than below the T Tauri band in the H-R diagram and have deduced rotational velocities comparable to those of typical T Tauri stars. Relatively rapid rotation in a late-type pre-main-sequence star is apparently not a sufficient condition for producing the strong line and continuum emission which characterizes true T Tauri stars.