Two unusual contact binaries in the globular cluster M54

Abstract

B, V and I light curves of V134 and B and V light curves of V144 have been analyzed by the W-D code. It is found that V134 is a middle mass ratio (q=0.530) deep contact binary with a contact degree of f=50.6% and V144 is an extremely low mass ratio (q=0.160) shallow contact binary with a contact degree of f=19.5%. The light curves of both of the two binaries show obvious O’Connell effect, this can be explained by employing dark spots on their secondary components. Based on the derived acceleration of gravity at the surface of the components, we found that the two contact binaries are very special, both components of the two binaries have possibly evolved off the main sequence and are likely to be subgiant stars. On the color-magnitude of M54, both of the two binaries are located in the blue straggler region. They are blue straggler candidates and may be formed by mass transfer.