Individual data sets of photometry of the post-nova X Ser. Panel a: photographic DASCH data (1932–1951). The events similar to those in Panel e are labeled 1 and 2. The short horizontal lines represent the upper limits of brightness. Panel b: histogram of brightness of the photographic data. Panel c: light curve from RoboScope (the V -band CCD data; 1992–1997; Honeycutt et al. 1998). The stunted outbursts are labeled H1, H2, and H3. Panel d: histogram of brightness of the RoboScope data. Panel e: light curve from Catalina Survey (2005–2013; circles) and CCD V -band AAVSO data (2009–2015; triangles). The outbursts investigated in Sect. 3.2 are labeled A to I. Panel f: histogram of brightness of the Catalina and AAVSO data (see Sect. 3 for details).