Abstract
Aims: The aim of this study is to investigate the physical properties of molecular envelopes of planetary nebulae (PNe) in their earliest stages of evolution. Methods: Using the 100 m telescope at Effelsberg, we have undertaken a high sensitivity discrete source survey for the first excited state of OH maser emission (J = 5/2, 2Π3/2 at 6 GHz) in the direction of PNe and proto-planetary nebulae (PNe) exhibiting 18 cm OH emission (main and/or satellite lines), and we further validate our detections using the Nançay radio telescope at 1.6-1.7 GHz and MERLIN interferometer at 1.6-1.7 and 6 GHz. Results: Two sources have been detected at 6035 MHz (5 cm), both of them are young (or very young) PNe. The first one is a confirmation of the detection of a weak 6035 MHz 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 1720 MHz OH emission. The detection of 6035 MHz OH maser emission is confirmed by subsequent observations made with the MERLIN interferometer. These lines are very rarely found in evolved stars. The 1612 MHz masers surround but are offset from the 1720 and 6035 MHz masers which in turn lie close to a compact 22 GHz continuum source embedded in the optical nebula.