Near-infrared time-series photometry of six fields in the young open cluster IC 239120 Oct 2013
Fields containing targets of spectral types later than M5 were monitored in JHKs for about 7 h each. None of the targets showed variability at levels exceeding 0.02 (or smaller, in some cases). Seven new variable stars were discovered serendipitously. The large number of measurements (more than 50) of each field allows the construction of accurate colour–magnitude and colour– colour diagrams. These could be used to identify a small number of candidate very cool objects.