  Lo, Sargent, and Young (1993) have recently concluded that the masses of some
dwarf galaxies, as deduced by the modified dynamics (MOND) from the observed
velocity dispersions, are systematically smaller than the observed masses, by a
factor of ten or more. We show here that the MOND mass estimator used by Lo et
al. is smaller than the proper expression, by a factor of about twenty. We
derive the proper mass estimator as an exact virial-like relation between the
3-D rms velocity, Vs, and the total mass, M, of an arbitrary, self-gravitating
system, made of light constituents, that is everywhere in the
very-low-acceleration regime of MOND. This reads M=(9/4)Vs^2/G a0. (For a
system that is not stationary, Vs involves also an average over time.) We
further generalize this relation to cases with constituent masses that are not
small compared with that of the whole system. We discuss various applications
of the M-V relation. With the correct estimator the predictions of MOND are, by
and large, in good agreement with the total observed masses (the observed gas
mass plus a stellar mass corresponding to an M/L of order one solar unit).
