1 Introduction

Emission from the first two rotationally excited states of OH was first discovered by Zuckerman et al. (1968) and Yen et al. (1969) for the $^{ 2}{ \Pi }_{ 1/2}{ }, J=1/2$ and $^{ 2}{ \Pi }_{3/2} , J=5/2$ states, respectively. The $^{ 2}{ \Pi }_{3/2} , J=5/2$ state of OH lies immediately above the ground-state and gives rise to four hyperfine transitions, with the F= 3-3 and 2-2 main lines at 6035.092 and 6030.747 MHz and the F= 3-2 and 2-3 satellite lines at 6049.084 and 6016.746 MHz, respectively (Fig. 1). The theoretical treatment of OH excitation in star-forming regions has progressed significantly in recent years (see Cesaroni & Walmsley 1991; Gray et al. 1992; Pavlakis & Kylafis 1996), and good predictions of relative OH line intensities can be made on the basis of these models, which show the importance of multi-line studies. In the circumstellar environment of late-type stars the model developed by Elitzur et al. (1976) successfully explains the excitation of strong 1612 MHz emission. This results from a cascade of the OH population down to the J= 1/2 and 3/2 states after far infrared photons at 34.6 micron and 53.3 micron (see Fig. 1) have excited the OH to the $^{ 2}{ \Pi }_{ 1/2}{ }, J=5/2$ and 3/2 states. There are enough far infrared photons to excite the 1612 MHz line (e.g. Epchtein et al. 1980). However, it is only recently that the direct detection of the 34.6 microns absorption line has been reported with the ISO telescope toward IRC+10420 (see Sylvester et al. 1997). Besides the conspicuous 1612 MHz line emission, 18 cm main line emission is often observed in late-type stars. Conditions for this emission are carefully investigated in the work of Collison & Nedoluha (1994, 1995) and we discuss later in this work the implication of their excitation mechanism for the J= 5/2 state of OH.

The main goal of the present observations was to survey the 5 cm $\Lambda$ doublet lines of OH in a number of stars ranging from typical Miras to OH/IR objects or pre-planetary nebulae. These stars sample various late stages of stellar evolution. In addition, observations of the $^{ 2}{ \Pi }_{3/2} , J=5/2$ state lying immediately above the ground-state provide a critical test for OH excitation models.

Figure 1: The energy level diagram for the $^{ 2}{ \Pi }_{ 3/2}$ and $^{ 2}{ \Pi }_{ 1/2}$ ladders of OH. $\Lambda$ doubling (not to scale) and parities are shown in each case. Transitions between the F=3 and 2 hyperfine levels, for $^{ 2}{ \Pi }_{3/2} , J=5/2$, give rise to the four 6 GHz lines.

These observations and our results obtained in 65 sources are presented in Sects. 2 and 3. Some OH properties of selected stars are also presented in Sect. 3. In Sect. 4 we discuss stellar OH pumping schemes and variability of OH main line emission sources in the light of our results.