  Relying on his photometry of galaxies, Holmberg (1958) found, more than 40
years ago, that the color indices of paired galaxies were closely correlated.
Our deep broad-band BVRI CCD photometry of 45 (S+S) pairs from the Karachentsev
(1972) catalogue (see Hernandez Toledo and Puerari, this volume), and
additional (B-V) color information from the literature (50 extra (S+S) pairs),
help us to confirm the effect. This "Holmberg Effect" has long been remained
unverified and not explained yet.
