EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars

This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT sample obtained at the 2.5 m Nordic Optical Telescope. The database consis ts of multi-epoch photo-polarimetry of 68 pre-main-sequence and main-sequen ce stars. An investigation of the polarization variability indicates that 2 2 objects are variable at the 3 sigma level in our data. All these objects are pre-main sequence stars, consisting of both T Tauri and Herbig Ae/Be ob jects while the main sequence, Vega type and post-T Tauri type objects are not variable. The polarization properties of the variable sources are mostl y indicative of the UXOR-type behaviour; the objects show highest polarizat ion when the brightness is at minimum. We add seven new objects to the clas s of UXOR variables (BH Cep, VX Cas, DK Tau, HK Ori, LkH alpha 234, KK Oph and RY Ori). The main reason for their discovery is the fact that our data- set is the largest in its kind, indicating that many more young UXOR-type p re-main sequence stars remain to be discovered. The set of Vega-like system s has been investigated for the presence of intrinsic polarization. As they lack variability, this was done using indirect methods, and apart from the known case of BD+31 degrees 643, the following stars were found to be stro ng candidates to exhibit polarization due to the presence of circumstellar disks: 51 Oph, BD+31 degrees 643C, HD 58647 and HD 233517.