双星与变星研究组知识库

Orbital period variations of some eclipsing cataclysmic variable stars (e.g., SW Sex-type CVs) and pre-CVs (e.g., HW Vir-type binaries) were analyzed by including some new times of light minimum obtained with several small telescopes in the world. It is found that the orbital periods of most sample stars show a cyclic change while they undergo a long-term variation. The evidence of magnetic breaking (NIB) for fully convective star was given. Our investigation indicates that the evolution of CVs with period shorter than 3 hours may be also governed by NIB, which is in agreement with new period distribution of CVs and with the results that fully convective stars are very active, with strong and stable magnetic field. It is revealed that the treat of MB in the standard theory should be modified and in this way the "period minimum" problem can be solved. The long-term period changes and the relation between the orbital period and its rate of change (dP/dt) are discovered for SW Sex-type CVs. These results supported that SW Sex-type CVs have the highest luminosity among CVs and was the reason that results in the observed SW Sextantis phenomena. We show that the cyclic period variations can not be explained by the mechanism of magnetic activity cycles of the late type component because of the problem of energy. The light-travel time effect via the presence of a tertiary component is the most possible mechanism to interpret the periodic change. It is shown that the extremely low-mass objects (e.g., brown dwarf stars and huge planets) are usually encountered as a tertiary component in CVs and pre-CVs.