Multi-Band Polarimetry of Be Stars. I. X Persei

We analyzed the multi-band polarimetric data of X Per during 1988--1994 around an extended low state in brightness. The variation in the intrinsic polarization correlated well with those of the Hα emission strength and the brightness variation. We give a rough estimate of the disk development, adopting a cylindrically symmetric sector. The disk becomes optically thick in its developed stage. It is a subtle problem whether a large photometric variation with Delta V=0.8 mag can be explained within the framework of the disk development, and is left as a challenging problem.