The low-period cutoff

Kolb's (1993) population synthesis calculations predict of the order of two times more CVs below the observed low period limit of CVs in general and classical novae in particular than above it. The predicted minimum period lying below the observed one can be explained by the insufficient knowledge of stellar opacities at low temperatures, rendering the calculated period cutoff highly uncertain. This issue is discussed by Kolb (1993) as well as by Rappaport et al. (1982).

Thus, the predicted presence of a large number of CVs below the observed period limit may just be due to an uncomplete understanding of CV evolution. However, this does not resolve the problem but only shifts it: These systems should then populate the range just above the empirical minimum period and thus the number of novae should be raised above the observed number. Therefore, compared to the overall synthetic CV population, classical novae appear to be deficient in the range between minimum period and period gap. However, as a caveat it must be mentioned, that on the observational side we are always dealing with small number statistics.