Narrow-band polarization measurements of late-type stars

Narrow-band polarization measurements at 4870 A and across the Ca II K line are reported for eight middle to late-type stars. For three stars (αCMi, αBOO and α1Cen) which are regularly used as unpolarized broad-band standards, measurements were made at +-10-4 accuracy or better. At 4870 A, null results were obtained and with this passband (10 A), these stars may be considered as narrow-band zero polarization standards. However, around the Ca II K region, the situation is different, there being a strong suspicion that αCMi exhibits a weak fluorescence polarization. A polarization dip at the K line in αSco has been detected with 95 per cent confidence. (author)