UBVRI photometry and polarimetry of the spotted red dwarf MS Ser

UBVRI photometry of the poorly studied spotted star HD 143313 (MS Ser) was carried out in 1991–1997, supplemented with polarization observations in 1996–1997. These observations confirmed the presence of BY Dra-type variability and reliably detected the star’s intrinsic polarization, which is variable in the U band. A model for the star’s spottedness that is consistent with the polarimetry data was constructed based on the collected photometric data. The model calculations suggest that the spotted areas cover up to 15% of the total stellar surface and are about 1500 K cooler than the surrounding photosphere.