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Abstract. The comparison of Hipparcos and FK5 proper motions points to an inconsistency with the correction, ∆p, of the luni-solar precession derived from VLBI and LLR observations with unprecedented accuracy. An attempt is made to explain this inconsistency of approximately −1.3 mas/yr by rotational offsets of the Hipparcos and FK5 proper motion systems. In terms of right ascension and declination, these offsets give rise to proper motion offsets in the range of ±1 mas/yr on average which is not exceptional given the FK5 error budget. In the case of Hipparcos it is proven that the proper motions are not affected by rotational offsets larger than those indicated by the errors of the proper motion link to the ICRF. This result is obtained by analysing the Hipparcos proper motions in view of the existence of additional systematic motions other than those caused by galactic rotation and the parallactic motion of stars due to the solar motion with respect to the LSR. It is concluded that the Hipparcos proper motions are nearly free of unmodelled rotations, confirming that the Hipparcos frame is inertial at the accuracy level of the proper motion link to the ICRF. The gap between the estimated precessional corrections is bridged primarily by minor changes in the FK5 proper motions of the order of their errors, and only to a small extent by the elimination of a bias in the Hipparcos proper motions.

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Abstract. Radio stars play a key role in establishing the link between optical reference frames and the conventional celestial reference frame based on extragalactic radio sources. The relevant astrometric, astrophysical and radio quantities are compiled of 66 cardinal radio stars currently suited to frame connection and maintenance of the link. The catalogue entries are supplied with ample bibliographical codes and annotations for easy data retrieval.1

ABSTRACT:After the processing of one year of observations carried out by HIP-PARCOS it is possible to provide quantitative results as to the number of new double and multiple stars to be detected and the real capacity of this mission to perform relative astrometry on double stars. We present and discuss the methods developed to this end and include the first results concerning the detection statistics and the determination of separation and position angle for double stars. About 16,000 stars have been recognized as non-single, including 9,000 already known as double and mutliple before the mission. Also, a subset of 10,500 stars have been successfully solved for their relative coordinates with an accuracy in the range of 3 to 10 mas.