PLANETARY-NEBULAE EXPANSION DISTANCES .3.

In a continuing project to determine accurate expansion-parallax dista nces to planetary nebulae, we have performed high-quality lambda 6-cm continuum observations of 12 nebulae with the Very Large Array. All of the sources in this imaging survey have one epoch of lambda 6-cm cont inuum data in the VLA archives. With two epochs for each planetary neb ula, we applied the expansion parallax algorithm. Due in large part to the poor quality of the preexisting epoch, we were successful in only one case (NGC 7662), and obtained a lower limit to the distance for t wo PNe (Hu 1-2 and NGC 2440). Here, we also present new images acquire d in this survey with the intent of reobserving the sample. Similar ob servations of these nebulae planned in 1997 should be sufficient to de tect the expansion of a large percentage of the nebular shells.