5 Conclusion

We have observed an extensive sample of OH/IR stars and late-type variables. Except for one atypical source (Vy 2-2) no excited OH maser has been detected. We do not confirm the tentative detection of NML Cyg. Only the blue shifted emission is detected in Vy 2-2 for which we have discussed briefly the morphology and OH production. The absence of detectable excited emission at 5 cm (except the unique object Vy 2-2) tends to argue in favor of a pumping scheme based essentially on the absorption of 35 and 53 $\mu$m photons. In the case of Vy 2-2 the ionization shell from where the maser emission seems to originate, may present physical conditions (shock, higher temperature and density) similar to those prevailing in HII regions. A hybrid pumping model applying to both OH/IR stars and HII regions or even a pumping scheme similar to OH maser emission in massive star forming regions may be successful. It is interesting to note that no absorption at 35 and 53 $\mu$m can be seen for this source in ISO observations. Interferometric observations are needed to spatially determine the likely related positions of the 1612 and 6035 MHz maser emission in Vy 2-2.

Acknowledgements

This research has made use of the SIMBAD database operated at CDS, Strasbourg, France and ASTRID database, operated at GRAAL, Montpellier, France. We thank Dr A. M. S. Richard for valuable suggestions to prepare the observations and N. J. Rodríguez-Fernández for his help on ISO data.