1. Introduction  
2. The VST-COSMOS data  
3. Selection of variable sources  
3.1. Catalog production and aperture selection  
3.2. Defects and masks  
3.3. The sample of AGN candidates  
4. Validation and characterization of the AGN candidates  
4.1. X-ray counterparts  
4.2. Spectroscopic and photometric classification  
4.3. Color-based classification  
5. Discussion