The poor cluster of galaxies S639
  We have studied the poor southern cluster of galaxies S639. Based on new Stromgren photometry of stars in the direction of the cluster we confirm that the galactic extinction affecting the cluster is large. We find the extinction in Johnson B to be A_B = 0.75+-0.03. We have obtained new photometry in Gunn r for E and S0 galaxies in the cluster. If the Fundamental Plane is used for determination of the relative distance and the peculiar velocity of the cluster we find a distance, in velocity units, of 5706+-350 km/s, and a substantial peculiar velocity, 839+-350 km/s. However, the colors and the absorption line indices of the E and S0 galaxies indicate that the stellar populations in these galaxies are different from those in similar galaxies in the two rich clusters Coma and HydraI. This difference may severely affect the distance determination and the derived peculiar velocity. The data are consistent with a non-significant peculiar velocity for S639 and the galaxies in the cluster being on average 0.2 dex younger than similar galaxies in Coma and HydraI. The results for S639 caution that some large peculiar velocities may be spurious and caused by unusual stellar populations. 
Inger Jorgensen (McDonald Observatory), Helge Jonch-Sorensen   (Copenhagen University Observatory)