tokens: ) within 3 arcsec of the new variables and (ii) manually check OGLE-II DIA light curves of nearby sources to make sure none of them show brightness trends mirroring the new variables. 3.6 Digitized photographic plates (66 Oph) Photographic images of the sky obtained in late 19th and 20th centuries contain a wealth of information about historical positions (e.g. Laycock et al. 2010 ; Vicente et al. 2010 ; Berezhnoi 2013 ; Robert et al. 2014 ) and brightness (e.g. Kolesnikova et al. 2008 ; Tang et al. 2013 ; Sokolovsky et al. 2014c ) of celestial objects. The efficient use of this information requires it to be converted to a digital form using purpose-built digitizing machines (Simcoe et al. 2006 ; De Cuyper et al. 2012 ) or a commercially available flatbed scanners capable of working with transparent materials (Vicente, Abad Garzón 2007 ; Simcoe 2009 ; Tuvikene et al. 2014 ). We use an Epson Expression 11000XL flatbed scanner operating at 2400 dpi resolution (1.4 arcsec pixel −1 , 16 bits per pixel colour depth) to digitize a 1.26 × 1.26 deg 2 area centred at α J2000 = 17:57:44.7 δ J2000 = +04:59:54 (66 Oph field; Kolesnikova et al. 2010 ) on 227 photographic plates obtained in 1976–1995 with the 40 cm astrograph. The digitized images were processed with vast following the procedure described by Sokolovsky et al. ( 2014b ). APASS B -band photometry of 1600 UCAC4 stars in the magnitude range B = 10–16 is used to calibrate the instrumental magnitude scale using the relation between aperture photographic and photoelectric magnitudes proposed by Bacher, Kimeswenger Teutsch ( 2005 ). We identify 23 variable stars including five not previously known (Table 4 , Fig. 2 ) by means of period search and visual inspection of light curves standing out in the magnitude versus σ w plot. 3.7 Light-curve filtering Often raw photometric data have to be pre-processed before computing the variability indices discussed in Section 2 . This may include (i) removing outliers from a light curve (possibly by applying iterative σ-clipping or median filtering); (ii) removing systematic effects from a set of light curves by applying local zero-point corrections (e.g. Nascimbeni et al. 2014 ) and/or the sysrem algorithm, decorrelating each light curve with external parameters such as airmass, seeing, object position on a CCD, detector temperature (e.g. Pál 2009 ; Bakos et al. 2010 ; López-Morales et al. 2010 ; Hartman et al. 2011 ; Burton et al. 2012 ; Guterman, Mazeh Faigler 2015 ; Baade et al. 2016 ) or de-trending the light curves if one is interested only in fast variability (e.g. Kovács, Bakos Noyes 2005 ; Weingrill 2015 ). A smaller-than-expected number of detections is an indirect indication of many photometry problems including the object being close to an image edge, a cosmetic defect, a bright star, a detection or saturation limit. Objects systematically affected by any of these problems can be removed from the analysis by discarding light curves having less than a
ner_tags: O O O O O O O O O O O O B-Survey I-Survey O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialRegion I-CelestialRegion I-CelestialRegion I-CelestialRegion I-CelestialRegion I-CelestialRegion I-CelestialRegion I-CelestialRegion B-CelestialObject I-CelestialObject O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O B-Instrument I-Instrument I-Instrument O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation B-Survey B-Wavelength I-Wavelength O O O B-Dataset O O O O O B-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O