Published online by Cambridge University Press: 12 April 2016
The celestial pole offsets (CPO) in provisional HIPPARCOS reference frame determined from the optical astrometry observations since 1899.7 until 1992.0 (Vondrák et al., 1996) and CPO determined from VLBI observations since 1980.0 until 1996.0 (Ma et al., 1996) are combined to get amplitudes of the long periodic nutation terms 18.6 and 9.3 years, respectively. The amplitudes are compared with previous solution based only on VLBI observations (Souchay et al., 1995).