Variable Extended Objects in SA 57

We have isolated a sample of 14 candidate variable objects with extended image structure to Bj = 22.5 in 0.284 deg^2 of Selected Area 57. The majority of 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 also compact and blue, and some of them are likely to be truly stellar. Twelve of the Bj <= 22.5 candidates have been observed spectroscopically over limited ranges of wavelength and a variety of resulting signal-to-noise. Five candidates display spectra consistent with Seyfert-like activity. In most cases where we have not been able to confirm a Seyfert spectroscopic type, the spectra are of insufficient quality or coverage to rule out such a classification. The majority of candidates have luminosities less than 10% of the nominal demarkation between QSOs and AGN (M(B) = -23). The surface density of 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 confirmed AGN in extended sources make up 36% of this population. Thus, the application of a variability criterion to images with extended structure enhances the completeness of the census of active nuclei. If the majority of our candidates are 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 sources contributing up to 33% of the total. (abridged)