THE LOWER MAIN-SEQUENCE AND THE ORBITAL PERIOD DISTRIBUTION OF CATACLYSMIC VARIABLE-STARS

The color-magnitude diagram of the lower main sequence, as measured fr om a volume-limited sample of nearby stars, shows an abrupt downward j ump between M-V similar to 12 and 13. This jump indicates that the obs erved mass-radius relationship steepens between 0.3 and 0.2 M-., but t heoretical models show no such effect. It is difficult to isolate the source of this disagreement: the observational mass-radius relationshi p relies upon transformations that may not be sufficiently accurate, w hile the theoretical relationship relies upon stellar models that may not be sufficiently complete, particularly in their treatment of the c omplex physics governing the interior equation of state. If the featur es in the observationally derived mass-radius relationship are real, t heir existence provides a natural explanation for the well-known gap i n the orbital period distribution of cataclysmic variables. This expla nation relies only upon the observed mass-radius relationship of low-m ass stars and does not require ad hoc changes in magnetic braking or i n the structure of cataclysmic variable secondaries. If correct, it wi ll allow broader application of cataclysmic variable observations to p roblems of basic stellar physics.