Abstract
The coexistence of ionized gas and OH maser emission is possible during the transition from red supergiant star to planetary nebul Under these circumstances the rcdshifted OH complex emi sion may be absorbed by the ionized core and will appear weaker than the blueshifted OH maser emission. Using the VLA at 6 and 2 - cm we searched for radio continuum emission in a list of OH/IR stars with such an asymmetric OH maser emission. No detections were made, and we conclude that these stars are still in the supenvind phase and that the OH asymmetry is caused by envelope asymmetries and not by the presence of an ionized core. A simple modeling of the transition from red supergiant star to planetary nebula suggests that the coexistence of radio continuum emission and OH maser emission lasts only a few tens of years. Since the superwind phase lasts a few thousand years, it is needed to survey a large sample (102 or more) of OH/IR stars to have a significant probability of detecting objects in the transition.
