| Abstract | These four binary systems are observed in the 21-cm neutral hydrogen line with the Westerbork Synthesis Radio Telescope (WSRT). All eight galaxies are detected; in the field of IC 65/UGC 622, two additional systems containing hydrogen are found. For these ten galaxies, systemic velocities and rotation curves are derived, and the total mass of each system is determined in two ways: (1) summing the results from the rotation curves and (2) calculating the minimum mass which follows from the orbital motion. In two of the four systems (NGC 3504/12 and IC 65/UGC 622), the minimum orbital mass is found to be larger than the mass deduced from the rotation curves, indicating the presence of invisible material. Results are then given of a 21-cm HI line study of five binary galaxies observed in the 21-cm HI line with the WSRT. All 10 galaxies are detected in HI, and systemic velocities and rotation curves are determined. The results are discussed in relation to a point mass model for the orbital motion in the pair. Only for the pair NGC 4016/4017 is a minimum orbital mass found, and this does not exceed the sum of the individual masses. For the other four pairs, the individual masses are not sufficient to explain the observed radial velocity differences. |
