  We present data of proper motions of OH masers in the massive star-forming
regions ON 1, K3-50, and W51 Main/South. OH maser motions in ON 1 are
consistent with expansion at approximately 5 km/s, likely tracing the expanding
ultracompact H II region. Motions in K3-50 are faster and may be indicating the
final stages of OH maser emission in the source, before the OH masers turn off
as the H II region transitions from the ultracompact to the compact phase. W51
South shows indications of aspherical expansion, while motions in W51 Main are
more difficult to interpret. Nevertheless, it appears that the relative
projected separation between W51 Main and W51 South is decreasing,
corresponding to an estimate of enclosed mass of at least 1500 solar masses,
consistent with estimates derived from millimeter-wavelength dust emission. We
confirm the ~20 mG magnetic fields previously seen in W51 Main, which may
represent the upper end of the density range allowable for 1665 MHz maser
emission. Magnetic field strengths and directions, obtained from Zeeman
splitting, in each source are consistent with values obtained in the first
epoch four to nine years ago.

Astrophysics