1 Introduction

The spatial relationship of interstellar OH and H$_{\rm 2}$O maser associations was investigated by Forster & Caswell (1989) - hereafter FC89) using VLA observations of 74 fields in the inner galaxy. The main aim of FC89 was to determine whether OH and H$_{\rm 2}$O masers coincide, and where they lie with respect to their exciting stars. The fields observed are a large sample of OH and H$_{\rm 2}$O masers with angular separation < 15$^{\prime\prime}$ in the galactic longitude range 335$^\circ$ to 50$^\circ$ and contains most of the southern maser associations known in 1985.

The VLA data showed that most OH and H$_{\rm 2}$O masers occur in small groups with diameters  Mpc (10¹⁷ cm, $\sim 6\, 000$ AU), and that the two groups generally coincide within the combined measurement errors ($\sim$1$^{\prime\prime}$ rms). A detailed correspondence of maser spots between the two species was not found however, and FC89 concluded that the OH and H$_{\rm 2}$O masers occur near a common energy source, but in physically distinct zones. When an Hii region is present the masers were found to lie within $\sim$20 mpc of the peak of the continuum emission, but not coincident with it.

Another important result of the FC89 survey is that the fields often contain several spatially distinct maser groups, not just a single OH/H$_{\rm 2}$O maser association. Altogether, 111 separate maser sites were found in the 74 fields mapped, including 36 isolated H$_{\rm 2}$O masers and 22 isolated OH masers. No Hii regions were found at the location of isolated H$_{\rm 2}$O masers, whereas four were found near isolated OH maser groups.

While the general properties of the sample as a whole were analyzed and discussed in FC89, the spectra and spatial distribution of the masers in each field were not given. We will henceforth refer to the complete VLA database of OH and H$_{\rm 2}$O masers, including spectra and spotmaps, as the FC89 database.

Subsets of the FC89 database have already been used by various authors to establish the connection between water masers and warm molecular cores (Codella et al. 1997), to compare the OH & H$_{\rm 2}$O maser positions with other maser species (e.g. Kraemer & Jackson 1995; Ellingsen et al. 1996), and to investigate the relationship of the masers to continuum sources (Carral et al. 1997). Works in progress which make use of the database include a deep search for continuum sources at maser sites (Forster & Caswell, in preparation), 3 mm molecular line observations toward isolated masers, and measurements of dust emission toward OH and H$_{\rm 2}$O masers.

In view of the number of requests for the VLA data we have decided to publish the FC89 database in electronic form so it will be easily accessible. We believe that it will be useful for finding the locations of dense cores in molecular clouds, in investigating the spatial and kinematical relation between the masers and their local environment, and in studies of the temporal evolution of masers in star-forming regions.