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Model-independent distance constraints to binary millisecond pulsars (MSPs)are of great value to both the timing observations of the radio pulsars, andmultiwavelength observations of their companion stars. Very Long BaselineInterferometry (VLBI) astrometry can be employed to provide thesemodel-independent distances with very high precision via the detection ofannual geometric parallax. Using the Very Long Baseline Array, we have observedtwo binary millisecond pulsars, PSR J1022+1001 and J2145-0750, over a two-yearperiod and measured their distances to be 700 +14 -10 pc and 613 +16 -14 pcrespectively. We use the well-calibrated distance in conjunction with revisedanalysis of optical photometry to tightly constrain the nature of their massive(M ~ 0.85 Msun) white dwarf companions. Finally, we show that severalmeasurements of their parallax and proper motion of PSR J1022+1001 and PSRJ2145-0750 obtained by pulsar timing array projects are incorrect, differingfrom the more precise VLBI values by up to 5 sigma. We investigate possiblecauses for the discrepancy, and find that imperfect modeling of the solar windis a likely candidate for the timing model errors given the low eclipticlatitude of these two pulsars.

Microarcsecond VLBI pulsar astrometry with PSRPI I. Two binary millisecond pulsars with white dwarf companions