On standard polarized stars

Intensive linear polarimetric measurements have been made to improve the standard polarized stars north of delta = -35 deg. Among the previously used standards, 55 Cyg (HD 198478), 9 Gem (HD 43384), and HD 183143 showed definite polarimetric variability on the time scale ranging from days to weeks. The amounts of change are in the order of 0.3% in p and/or greater than 1 deg in the position angles. These three stars are therefore not suitable as standards for accurate polarimetric observations. The incidence of the short time-scale polarimetric variability seems to be higher among the supergiants of earlier spectral type than those of later spectral type. The polarization values of some of the other standard polarized stars are determined to an estimated accuracy of + or - 0.01% in p and + or - 0.2 deg in the position angles.