Starspot photometry with robotic telescopes Continuous UBV and V (RI)C photometry of 23 stars in 1991{1996

Abstract. We report on the progress of our ongoing pho-tometric monitoring program of spotted late-type stars with automatic photoelectric telescopes (APTs) on Mt. Hopkins in Arizona and on Mt. Etna in Sicily. We present 9 250 dierential UBV and/or V (RI)C observations for altogether 23 chromospherically active stars, singles and binaries, pre main sequence and post main sequence, taken between 1991 and 1996. The variability mechanism of our target stars is mostly rotational modulation by an asym-metrically spotted stellar surface. Therefore, precise rota-tional periods and their seasonal variations are determined using baselines between 3 years for HD 129333 to 34 years for V410 Tauri. We report the largest V light-curve am-plitude of any spotted star observed to date: 0:m65 for V410 Tau in 1994{95. Long-term variations of the over-all light levels of our target stars are sometimes of similar amplitude as the rotational modulation itself and are most likely caused by an analog of the solar 11-year spot cycle but mostly without a well-dened periodicity. For some o