ANOMALOUS WATER MASERS IN OH IR STARS

We present OH (1612 MHz) and H2O (22.2 GHz) maser observations made wi th the VLA at an angular resolution of approximately 1'' toward three OH/IR stars: OH 12.8-0.9, OH37.1-0.8 and OH42.3-0.1. These stars exhib it the H2O maser components in a velocity range that exceeds the OH ve locity range, a situation in disagreement with the standard model for maser emission in late-type stars. We find that the OH and H2O maser p ositions agree within approximately 0.5'' and that most likely both ma sers arise from the same source, ruling out the possibility of a fortu itous superposition of two different sources. We discuss some of the p ossible mechanisms that could explain the anomalous H2O maser emission . The most likely explanation is that these stars have axisymmetric wi nds and that the H2O emission originates in the polar regions with lar ger velocities than the regions closer to the equator where the OH emi ssion arises. Very long baseline observations are required to test thi s explanation.