  The aim of this study is to investigate the physical properties of molecular
envelopes of planetary nebulae in their earliest stages of evolution. Using the
100m telescope at Effelsberg, we have undertaken a high sensitivity discrete
source survey for the first excited state of OH maser emission (J=5/2, 2PI3/2
at 6GHz) in the direction of planetary and proto-planetary nebulae exhibiting
18cm OH emission (main and/or satellite lines), and we further validate our
detections using the Nan\c{c}ay radio telescope at 1.6-1.7GHz and MERLIN
interferometer at 1.6-1.7 and 6GHz. Two sources have been detected at 6035MHz
(5cm), both of them are young (or very young) planetary nebulae. The first one
is a confirmation of the detection of a weak 6035MHz line in Vy 2-2. The second
one is a new detection, in K 3-35, which was already known to be an exceptional
late type star because it exhibits 1720MHz OH emission. The detection of
6035MHz OH maser emission is confirmed by subsequent observations made with the
MERLIN interferometer. These lines are very rarely found in evolved stars. The
1612MHz masers surround but are offset from the 1720 and 6035MHz masers which
in turn lie close to a compact 22GHz continuum source embedded in the optical
nebula.
