The lower main sequence and the orbital period distribution of cataclysmic variable stars

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 M-V similar to 12 and 13. This jump indicates that the observed mass-radius relationship steepens between 0.3 and 0.2 M-., 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.