摘要：In this paper, we present the first light curve synthesis and orbital period change analysis of nine contact binaries around the short-period limit.It is found that all these systems are W-subtype contact binaries.One of them is a medium contact system while the others are shallow contact ones.Four of them manifest obvious O'Connell effect explained by a dark spot or hot spot on one of the component stars.Third light was detected in three systems.By investigating orbital period variations, we found that four of the targets display a secular period decrease while the others exhibit a long-term period increase.The secular period decrease is more likely caused by angular-momentum loss while the long-term period increase is due to mass transfer from the less massive component to the more massive one.Based on the statistic of 19 ultrashort-period contact binaries with known orbital period changes, we found that seven of them display long-term decrease (three of them also exhibit cyclic variations), ten of them manifest long-term increase while two of them only show cyclic variation, and that most of them are shallow contact binaries supporting the long timescale angular-momentum loss theory suggested by Stepien.For the three deep contact systems, we found that they are probably triple systems.The tertiary companion plays an essential role during their formation and evolution.
