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We have devised a method to select galaxies that are isolated in their darkmatter halo (N=1 systems) and galaxies that reside in a group of exactly two(N=2 systems). Our N=2 systems are widely-separated (up to$\sim$\,200\,$h^{-1}$\,kpc), where close galaxy-galaxy interactions are notdominant. We apply our selection criteria to two volume-limited samples ofgalaxies from SDSS DR6 with $M_{r}-5 \log_{10} h \leq$ -19 and -20 to study theeffects of the environment of very sparse groups on galaxy colour. Forsatellite galaxies in a group of two, we find a red excess attributed to starformation quenching of 0.15\,$\pm$\,0.01 and 0.14\,$\pm$\,0.01 for the -19 and-20 samples, respectively, relative to isolated galaxies of the same stellarmass. Assuming N=1 systems are the progenitors of N=2 systems, animmediate-rapid star formation quenching scenario is inconsistent with theseobservations. A delayed-then-rapid star formation quenching scenario with adelay time of 3.3 and 3.7\,Gyr for the -19 and -20 samples, respectively,yields a red excess prediction in agreement with the observations. Theobservations also reveal that central galaxies in a group of two have a slightblue excess of 0.06\,$\pm$\,0.02 and 0.02\,$\pm$\,0.01 for the -19 and -20samples, respectively, relative to N=1 populations of the same stellar mass.Our results demonstrate that even the environment of very sparse groups ofluminous galaxies influence galaxy evolution and in-depth studies of thesesimple systems are an essential step towards understanding galaxy evolution ingeneral.

Groups of two galaxies in SDSS: implications of colours on star formation quenching time-scales