Examples of lightcurves of four candidates identified by WAVDETECT, constructed via forced aperture photometry on the difference images. The lightcurve in the top left panel is that of an artifact resulting from imperfect PSF-matching in the DI, while the one in the right panel is that of a nova (cf. Sect. 6). The two lightcurves in the bottom are that of genuine variable sources. For the source in the bottom right panel, all the measured fluxes have negative values. This occurs because the reference image used in the difference imaging is not an unbiased average of the random phases of the lightcurve.