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.