  We present the correlation between the extrapolated central disk surface
brightness (mu) and extrapolated central surface mass density (Sigma) for
galaxies in the DiskMass sample. This mu-Sigma-relation has a small scatter of
30% at the high-surface-brightness (HSB) end. At the low surface brightness
(LSB) end, galaxies fall above the mu-Sigma-relation, which we attribute to
their higher dark matter content. After correcting for the dark matter, as well
as for the contribution of gas and the effects of radial gradients in the disk,
the LSB end falls back on the linear mu-Sigma-relation. The resulting scatter
about the corrected mu-Sigma-relation is 25% at the HSB end, and about 50% at
the LSB end. The intrinsic scatter in the mu-Sigma-relation is estimated to be
10% to 20%. Thus, if the surface brightness is known, the stellar surface mass
density is known to within 10-20% (random error). Assuming disks have an
exponential vertical distribution of mass, the average (M_L)_K is 0.24
Msun/Lsun, with an intrinsic scatter around the mean of at most 0.05 Msun/Lsun.
This value for (M/L)_K is 20% smaller than we found in Martinsson et al.,
mainly due to the correction for dark matter applied here. This small scatter
means that among the galaxies in our sample variations in scale height,
vertical density profile shape, and/or the ratio of vertical over radial
velocity dispersion must be small.
