Variable Extended Objects in Selected Area 57

We have isolated a sample of 14 candidate variable objects with extendedimage structure to Bj = 22.5 in 0.284 deg^2 of Selected Area 57. The majorityof candidates are blue (U-B<0) and relatively compact. At fainter magnitudes,there is a steep rise in the number of variable objects. These objects are alsocompact and blue, and some of them are likely to be truly stellar. Twelve ofthe Bj <= 22.5 candidates have been observed spectroscopically over limitedranges of wavelength and a variety of resulting signal-to-noise. Fivecandidates display spectra consistent with Seyfert-like activity. In most caseswhere we have not been able to confirm a Seyfert spectroscopic type, thespectra are of insufficient quality or coverage to rule out such aclassification. The majority of candidates have luminosities less than 10% ofthe nominal demarkation between QSOs and AGN (M(B) = -23). The surface densityof confirmed M(B) > -23 AGN to Bj = 22, including stellar sources, is~40/deg^2, in good agreement with other surveys at this depth. The confirmedAGN in extended sources make up 36% of this population. Thus, the applicationof a variability criterion to images with extended structure enhances thecompleteness of the census of active nuclei. If the majority of our candidatesare bona fide AGN, the surface density could be as high as 82/deg^2 for M(B) >-23, and 162/deg^2 for all luminosities to Bj = 22, with extended sourcescontributing up to 33% of the total. (abridged)