SIMULTANEOUS MULTICOLOR PHOTOMETRIC AND POLARIMETRIC OBSERVATIONS OF 4 T-TAURI STARS

We present simultaneous multicolour photometric and polarimetric obser vations of the T Tauri stars RY Tau, V1121 Oph, DI Cep and BM And. Sig nificant variabilities on time-scales of days in the degree of polariz ation and/or in the position angle were recorded for DI Cep and BM And . None of the stars revelled changes in the polarization properties on time-scales shorter than one day. One star, BM And, showed anti-corre lations between its brightness level and degree of polarization, consi stent with a period of similar to 4 days. The change of its position a ngle of similar to 25 degrees implies the presence of a strongly varia ble intrinsic polarization relative to a constant interstellar polariz ation component. We speculate that it is caused by irradiation of abet spot. The presence of a hot source is also indicated by the photometr ic measurements. DI Cep increased its brightness by 0.5 magnitudes dur ing four nights while its degree of polarization and the position angl e showed only modest variations. V1121 Oph showed a wavelength depende nt rotation of its polarization vector with an increase in the positio n angle of similar to 43 degrees from the U band to the I band.