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We study a sample of 23 narrow-emission line galaxies (NELGs) which wereselected by their strong variability as QSO candidates in the framework of avariability-and-proper motion QSO survey on digitised Schmidt plates. Inprevious work, we have shown that variability is an efficient method to findAGNs. The variability properties of the NELGs are however significantlydifferent from those of the QSOs. The main aim of this paper is to clarify thenature of this variability and to estimate the fraction of AGN-dominated NELGsin this sample. New photometric and spectroscopic observations are presented,along with revised data from the photographic photometry. The originallymeasured high variability indices could not be confirmed. The diagnosticline-ratios of the NELG spectra are consistent with HII region-like spectra. NoAGN could be proved, yet we cannot rule out the existence of faintlow-luminosity AGNs masked by HII regions from intense star formation.Comment: 10 pages, 8 figures, accepted for publication in Astronomy &  Astrophysic

A QSO survey via optical variability and zero proper motion in the M 92 field