BI VULPECULAE: A SIAMESE TWIN WITH TWO VERY SIMILAR COOL STARS IN SHALLOW CONTACT

BI Vul is a cool eclipsing binary star with a short period of 0.2518 days. The first charge-coupled device (CCD) light curves of the binary in the BVRI obtained on 2012 August 21 are presented and are analyzed using the Wilson-Devinney code. It is discovered that BI Vul is a marginal contact binary system that contains two very similar cool components. Both the marginal contact configuration and the extremely high mass ratio suggest that it is presently evolving into contact with little mass transfer between the components and it is at the beginning stage of contact evolution. The period decrease may be caused by angular momentum loss via magnetic stellar wind and/or it is only a part of a long-period cyclic variation, which may reveal the presence of another stellar companion in a wider orbit. These observational properties indicate that the formation of the Siamese twin is driven by magnetic braking and the third stellar companion should play an important role by removing angular momentum from the central binary.