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The color-magnitude diagram of the lower main sequence, as measured from avolume-limited sample of nearby stars, shows an abrupt downward jump between Mv= 12 and 13. This jump indicates that the observed mass-radius relationshipsteepens between 0.3 and 0.2 solar masses, but theoretical models show no sucheffect. It is difficult to isolate the source of this disagreement: theobservational mass-radius relationship relies upon transformations that may notbe sufficiently accurate, while the theoretical relationship relies uponstellar models that may not be sufficiently complete, particularly in theirtreatment of the complex physics governing the interior equation-of-state.  If the features in the observationally derived mass-radius relationship arereal, their existence provides a natural explanation for the well-known gap inthe orbital period distribution of cataclysmic variables. This explanationrelies only upon the observed mass-radius relationship of low-mass stars, anddoes not require ad hoc changes in magnetic braking or in the structure ofcataclysmic variable secondaries. If correct, it will allow broader applicationof cataclysmic variable observations to problems of basic stellar physics.Comment: 27 pages, 10 postscript figures, AASTeX (aaspp4), accepted by Ap

The Lower Main Sequence and the Orbital Period Distribution of Cataclysmic Variable Stars