We consider as M51-type the following pairs: Arp 36, Arp 70,
Arp 74, Arp 82, Arp 86, Arp 87, Arp 183, Arp 218,
Kar 64, Kar 168,
Kar 296, Kar 302 and Kar 404. It is worth noticing that not only
large galaxies, like Arp 86, but also rather small ones like
Kar 302 can have well developed, strong spiral arms.
Arp 298 is considered marginal, because the spiral arms
are not as strong and deeply penetrating as for the other
members of this category, being
more ring-like. The main
galaxy is classified as a Seyfert 1. It has a nuclear
ring in B-V, as found earlier in optical broad band images
by Buta 
Crocker (1993). This is an interesting
characteristic because, contrary to Seyfert 2 galaxies,
Seyfert 1 nuclei are not generally found to have extended
nuclear starbursts.
Although the redshifts of the companions are not known for Arp 70,
Arp 74 and Arp 36, and for neither of the components of Arp 218,
interpreting them as interacting pairs is clear on morphological
bases. However, for Arp 183, which also lack redshift measurement
the interaction is less clear, since the presumed companion
is over 4 magnitudes fainter.
Incidentally, Arp 36 resides at a distance of 8 galaxy diameters
from Arp 183 and has approximately the same recession velocity,
but for its large separation most probably does not have any
significant current influence on Arp 183.
Altogether, 8 out of 13 M51-type pairs with sufficient information (excluding Arp 298), have blue B-V colors in the central regions of the companions (Arp 82, Arp 86, Kar 64, Kar 168, Arp 87, Kar 302, Arp 36, and Kar 404) and only one of them in the main galaxy (Arp 74). The large number of active companions is quite suprising because gas content of normal galaxies is not able to sustain long-lived starburst activity. For example, in NGC 7752, the companion of Arp 86, a starburst of the present strength would consume all gas of its disk in about 200 Myr (Laurikainen et al. 1993). However, if there is mass transfer from the main galaxy to the companion, this may have triggered a nuclear starburst or extended star formation in the central regions of the companion, and provide the required source of gas. In some of the companion galaxies, like in Arp 87 or Arp 82, the blue central regions are partly obscured either by dust or the red stellar component. Three of the above pairs (Arp 82, Arp 87 and Kar 302) show knotty star forming regions in the tail-arm opposite to the companion and one of them (Kar 296) in the bridge arm (see the R-I maps). Arp 87 is listed as a possible candidate for polar-ring galaxies by Whitemore et al. (1990). It is also worth noticing that the companion in Kar 404 has extremely blue spiral arms.
The pairs that do not have blue B-V colors in the central regions of
the companions are Arp 70, Arp 183, Arp 218, Arp 74 and Kar
296. However, Arp 70 has small nuclear R-I color, which hints to
recent star formation: the small R-I value is supposed to be due to
strong H
and NII emission lines, emanating through the
continuum. This rises the number of companions with enhanced star
formation to 9. For Kar 296 one of the inner spiral arms of the
companion has blue B-V color. Except for Kar 296, the above pairs show
more dispersed spiral structures than the M51-type pairs of our sample
in general. Additionally, Arp 70 and Arp 218 have warps thus
indicating that the companions may not be orbiting in the plane of the
main galactic disk.
Arp 36 is classified as an M51-type system by Vorontsov-Velyaminov (1977) and is suggested to show a bar and a double companion at the end of the single developed arm. We also consider Arp 36 as M51-type system, but for a different reason: which to Vorontsov-Velyaminov is a bar, appears to us a system of two galactic nuclei, both connected to their own spirals, and a strong HII-region. Due to their R-I colors the knots at the end of the well developed arms are most probably strong HII-regions, not galaxies as suggested by Vorontsov-Velyaminov. The smaller galaxy in the system (about 10'' N-W) has blue nuclear B-V color. This is an interesting system, because it may represent an example of M51-type pairs in a late phase of evolution. Similarly, Arp 183 has very dispersed spiral arms.
The remaining non M51-type pairs show large-scale instabilities some of them manifesting as strong color gradients in their disks. The main characteristics for some individual pairs are listed below:
Kar 203 and Kar 125 are spiral-spiral pairs with galaxy separations of 1-1.5 main galaxy diameters. All four galaxies show morphological peculiarities and the spiral arms are rather dispersed and irregular. At least one galaxy in both pairs shows strong B-V color gradient throughout the disk (Kar 203 B and Kar 125 A; for Kar 203 A we do not have color maps). The R-I images have no noticable gradients, but instead show knotty star forming regions in the same galaxies.
Kar 471 B, like Kar 203 B and Kar 125 A, has a B-V gradient and knotty star forming regions. However, in this case the color gradient is rather a color difference between the two arms rather than a gradient through the disk. The companion, Kar 471 A, has strong one-sided star forming region in the inner disk (see the R-I image).
Kar 523 is a pair of two barred galaxies. Kar 523 B has a dust lane on the right side between the outer arm and part of the inner pseudoring. There is also star formation along the bar and in a small region in the nucleus, detected in B-V. In the R-I image one can also see a nuclear ring, about 5'' in diameter, but it might be an artefact caused by seeing. However, rebinning the images by a factor of 4 left the ring still visible.
NGC 5905/5908. NGC 5905 is a barred galaxy with a nuclear ring, about 5'' in diameter, detected in B-V, found earlier by Wozniak et al. (1995).
Kar 331 is an exception in our sample because both galaxies are lenses, otherwise it looks like an M51-type pair. Kar 331 A has a nuclear ring visible in B-V, about 10'' in diameter, and the isophotes of the galaxy are twisted. The bright nearby object in the images of Kar 331 A is a star.
Acknowledgements
E. Laurikainen and H. Salo wish to thank Instituto de Astronomia UNAM (Mexico) for its hospitality during their visit, when part of the reductions were carried out. Special thank to Irene Cruz-González, Margarita Rosado, Deborah Dultzin and Etienne Le Coarer for their interest to this project which led to collaboration in Fabry-Perot and IR-observations. We also thank Prof. G. Paturel for his most valuable comments.