  The color-magnitude diagram of the lower main sequence, as measured from a
volume-limited sample of nearby stars, shows an abrupt downward jump between Mv
= 12 and 13. This jump indicates that the observed mass-radius relationship
steepens between 0.3 and 0.2 solar masses, but theoretical models show no such
effect. It is difficult to isolate the source of this disagreement: the
observational mass-radius relationship relies upon transformations that may not
be sufficiently accurate, while the theoretical relationship relies upon
stellar models that may not be sufficiently complete, particularly in their
treatment of the complex physics governing the interior equation-of-state.
  If the features in the observationally derived mass-radius relationship are
real, their existence provides a natural explanation for the well-known gap in
the orbital period distribution of cataclysmic variables. This explanation
relies only upon the observed mass-radius relationship of low-mass stars, and
does not require ad hoc changes in magnetic braking or in the structure of
cataclysmic variable secondaries. If correct, it will allow broader application
of cataclysmic variable observations to problems of basic stellar physics.
