  Despite of numerous efforts, the stage from Asymptotic Giant Branch (AGB) to
Planetary Nebulae (PN) is a poorly understood phase of stellar evolution. We
have therefore carried out interferometric (VLA) observations of a sample of
hot post-AGB stars, selected on the basis of their optical and infrared
properties. Ten sources, out of the 16 been observed, were detected. This
indicates that most of our targets are surrounded by a nebula where the
ionization has already started. This definitively determines the evolutionary
status of the selected sources and provides us with a unique sample of very
young Planetary Nebulae (yPNs).
