VLBI ASTROMETRY OF THE MILLISECOND PULSAR B1937+21

We report the results of astrometric VLBI observations of PSR B1937+21 . Observations of the pulsar and a nearby quasar were made at 1.67 GHz , using four antennas: Arecibo, the very large array, and the DSN 70 m antennas in California and Spain. We determine a position of alpha=19 (h)39(m)38(s).5611+/-0(S).0003 and delta=21 degrees 34'59''.118+/-0''. 016 for the pulsar in the IERS extragalactic reference frame at epoch 1990.2. The accuracy of our result in right ascension is limited by sy stematic uncertainties in the ionospheric delay calibration, and large r-than-expected random scatter in the pulsar delays related to interst ellar scintillation. The accuracy in declination is limited by the lac k of a long north-south baseline in our experiment and the uncertainty of the location of the Arecibo antenna in the IERS celestial referenc e frame. When compared with the pulsar's timing position [Kaspi, et al ., ApJ, 428, 713 (1994)] in the planetary ephemeris reference frame, o ur result yields a measurement of the offset of the IERS reference fra me and the DE200 planetary ephemeris frame of Delta alpha((IERS-DE200) )=0(s).0008+/-0(s).0003 and Delta delta((IERS-DE200))=-0''.024+/-0''.0 16, which is in reasonable agreement with the frame-tie rotation deter mined by Folkner et al. [ARA&A, 297, 279, (1994)]. (C) 1996 American A stronomical Society.