Extended HI disks in nearby spiral galaxies
  In this short write-up, I will concentrate on a few topics of interest. In
the 1970s I found very extended HI disks in galaxies such as NGC 5055 and NGC
2841, out to 2 - 2.5 times the Holmberg radius. Since these galaxies are
warped, a "tilted ring model" allows rotation curves to be derived, and
evidence for dark matter to be found. The evaluation of the amount of dark
matter is hampered by a disk-halo degeneracy, which can possibly be broken by
observations of velocity dispersions in both the MgI region and the CaII
region.
