Hideo Sato National Astronomical Observatory, Japan
Keiichi Saijo National Science Museum, Japan
Kazuo Yoshioka The University of the Air, Japan
Janet A. Mattei* AAVSO, 25 Birch St, Cambridge, MA 02138, USA
To investigate irregular variations in brightness and in period change of
long period variable stars (LPVs), we made statistical studies of period variations
of such stars, based on two- and three-dimensional return maps and smoothed
diagrams using spline curves on the period variations of these stars.