Previously known serendipitous sources were selected by cross-correlating WGACAT with a variety of optical and radio catalogs, including the Verón-Cetty & Verón (1996) and Hewitt & Burbidge (1993) quasar catalogs, the 1 Jy (Stickel et al. 1994) and S4 (Stickel & Kühr 1994) radio catalogs, and the BL Lac catalog of Padovani & Giommi (1995b). Also, the classification of non-AGN sources (which is important to select the unclassified objects) was done as described in White et al. (1995). In some cases classifications were also double-checked in the NASA Extragalactic Database (NED) or taken from very recent papers. All objects broad-line radio galaxies (BLRGs) have been included with the FSRQ class (see §3). The adopted cross-correlation radii were the same as those used for the selection of our candidates (§ 2). Selection criteria identical to those described in §2 were applied to the previously known objects. Potential mis-identifications through chance coincidences of previously known AGN which fulfill our selection criteria were addressed by shifting the X-ray positions by one degree at a time several times, and repeating the cross-correlations between WGACAT and the AGN catalogs. The number of spurious X-ray/optical associations was zero, which implies a negligible (2 upper limit ) contamination by spurious sources.
Ninety-seven previously known objects were thus found, of which 11 are BL Lacs and 76 are FSRQs. The remaining ten objects are Narrow Line Radio Galaxies (NLRGs) with . Details of these 97 objects are given in Table 6. The mean X-ray/optical offset for these sources is arcsec, while the median one is arcsec, in agreement with the estimated errors on the positions of the WGA sources (§ 2). Since no preselection was done on WGACAT for these objects (with the exception of excluding regions at and within of the LMC, SMC and M31), these objects will qualify for our complete sample, and will be discussed along with the newly-identified sources in §§5-7.