  We report on recent results from our successful and pioneering observational
program with ALMA to study emerging ultracom pact HII regions of pre-Planetary
Nebulae (pPNe) using mm-wavelength recombination lines (mRRLs) as new optimal
tracers. We focus on our study of two poster-child pPNe, namely, M2-9 and
CRL618. We reveal the structure and kinematics of the en igmatic inner nebular
regions of these objects with an unprecedented angular resolution down to
20-30mas (~15-30AU). For both targets, the ionized central regions are
elongated along the main symmetry axis of the large-scale nebulae, consiste nt
with bipolar winds, and show notable axial velocity gradients with expansion
velocities of up to ~100km/s. The H30a pr ofiles exhibit time variability,
reflecting changes in the physical properties and kinematics on scales of a few
years. O ur ongoing analysis employs 3D, non-LTE radiative transfer modeling,
providing a detailed description of the innermost la yers of these well known
pPNe with exceptional clarity.
