Trigonometric parallax measurements of post-AGB stars with VERA

A trigonometric parallax distance is measurable for a planetary nebula if it harbors H₂O (or OH) maser spots. We have demonstrated it for H₂O maser sources, K3-35, IRAS 19312+1950 and IRAS 18286-0959 using the VLBI Exploration of Radio Astrometry (VERA). They are post-AGB stars and exhibit peculiar morpho-kinematical structures like a bipolar planetary nebula or a bipolar molecular jet (water fountain). The luminosities and secular motions of these sources derived from the trigonometric distances and proper motions of the H₂O masers suggest that their parental stars may be intermediate-mass (2M _⊙ < M _* < 8 M _⊙) evolved stars.