We compare the parallaxes of stars from VLBI astrometry in the literature to those in the Gaia DR2 catalog. Our full sample contains young stellar objects, evolved AGB stars, pulsars and other radio stars. Excluding AGB stars, which show significant discrepancies between Gaia and VLBI parallaxes, and stars in binary systems, we obtain an average, systematic, parallax offset of −75 ± 29 µas for Gaia DR2, consistent with their estimate of a parallax zero-point between −100 and 0 µas.

We report astrometric results of VLBI phase-referencing observations of 22 GHz H 2 O masers emission toward the red hypergiant VX Sgr, one of most massive and luminous red hypergiant stars in our Galaxy, using the Very Long Baseline Array. A background source, J1820−2528, projected 4°.4 from the target VX Sgr, was used as the phase reference. For the low decl. of these sources, such a large separation normally would seriously degrade the relative astrometry. We use a two-step method of tropospheric delay calibration, which combines the VLBI geodetic-block (or Global Positioning System) calibration with an image-optimization calibration, to obtain a trigonometric parallax of 0.64±0.04 mas, corresponding to a distance of -+ 1.56 0.10 0.11 kpc. The measured proper motion of VX Sgr is 0.36±0.76 and −2.92±0.78 mas yr −1 in the eastward and northward directions. The parallax and proper motion confirms that VX Sgr belong to the Sgr OB1 association. Rescaling bolometric luminosities in the literature to our parallax distance, we find that the luminosity of VX Sgris (1.95±0.62)×105 L e , where the uncertainty is dominated by differing photometry measurements.

We report Very Long Baseline Array observations of 22 GHz H2O and 43 GHz SiO masers toward the Mira variable RR Aql. By fitting the SiO maser emission to a circular ring, we estimate the absolute stellar position of RR Aql and find agreement with Gaia astrometry to within the joint uncertainty of ≈1 mas. Using the maser astrometry we measure a stellar parallax of 2.44 ± 0.07 mas, corresponding to a distance of 410 − 11 + 12 pc. The maser parallax deviates significantly from the Gaia EDR3 parallax of 1.95 ± 0.11 mas, indicating a 3.8σ tension between radio and optical measurements. This tension is most likely caused by optical photocenter variations limiting the Gaia astrometric accuracy for this Mira variable. Combining infrared magnitudes with parallaxes for RR Aql and other Miras, we fit a period–luminosity relation using a Bayesian approach with Markov Chain Monte Carlo sampling and a strong prior for the slope of −3.60 ± 0.30 from the Large Magellanic Cloud. We find a K-band zero-point (defined at logP(days) = 2.30) of −6.79 ± 0.15 mag using very long baseline interferometry (VLBI) parallaxes and −7.08 ± 0.29 mag using Gaia parallaxes. The Gaia zero-point is statistically consistent with the more accurate VLBI value.