  Upper limits for the disk mass-to-light ratios for the low surface brightness
galaxies UGC 6614 and F568-6 (Malin 2) are estimated by considering the minimum
velocity perturbations in the HI velocity field that should result from the
spiral structure observed in the R band images. The weak observed response in
the $\phi$ velocity component limits the mass-to-light ratios of the disk
within a scale length to M/L <~ 3 and 6 for UGC 6614 for F568-6 respectively
(in solar units) based upon azimuthal variations observed in the R band images.
These limits are sufficiently strong to require a significant dark matter
component even in the central regions of these galaxies. Our limits furthermore
imply that this dark matter component cannot be in the form of a cold disk
since a cold disk would necessarily be involved in the spiral structure.
However, a more massive disk could be consistent with the observations because
of a non-linear gas response or if the gas is driven by bar-like distortions
instead of spiral structure. To produce the large observed arm/interarm HI
density variations it is likely that the spiral arm potential perturbation is
sufficiently strong to produce shocks in the gas. For a forcing that is greater
than 2% of the axisymmetric force, M/L >~ 1 is required in both galaxies in the
outer regions. These lower limits imply that the stellar surface density is at
least of the same order as the gas surface density. This is consistent with the
large scale morphology of the spiral structure, and the stability of the gas
disk, both which suggest that a moderate stellar component is required to
produce the observed spiral structure. (Shortened abstract)
