The minimum period and the gap in periods of cataclysmic binaries

It is regarded as almost certain that the upper boundary of the gap in orbital periods observed at about 3 hours is due to enhanced angular momentum losses from cataclysmic binaries that have longer periods. The role that stellar winds may play here is discussed. It is noted that the lower boundary of the gap can be explained with the oldest cataclysmic binaries, the periods of which evolved past the minimum at 81 minutes and reached the value of 2 hours within about 12 x 10 to the 9th years after the binary had formed. Those binaries should have secondary components of only 0.02 solar mass, and their periods could be used to estimate the ages of the oldest cataclysmic stars and, presumably, the age of the Galaxy. An alternative explanation for the gap requires that binaries be detached while crossing the gap. A mechanism which may be responsible for this phenomenon is considered. It requires the secondary components to be approximately 0.2 solar mass in the binaries just below the gap.