Photometric parameters, distance and period-colour study of contact binary stars in the globular cluster ! Centauri

We present V passband photometric solutions of six contact binaries in the globular cluster ! Centauri using the Wilson-Devinney code. We found that four of the systems are H subtype W UMa binary systems and remaining two are W-subtype W UMa systems. Four of the systems show the O'Connell effect in their light curves and brightness at 0.75 phase is about -V=0.04i0.05 less than at 0.25 phase. We show that one dark/cool spot solution on secondary companion satisfactorily fits the observed light curves for the respective contact binaries. The derived spot values are not unique and they are taken to be tentative at best. The colour index of the six contact binaries were used to derive the distance modulus of ! Centauri and found an average value of »13.60 which is close to the observed value of 13.97. We also derive the distance to each of the studied variables. The period-colour relation of the contact binaries of ! Centauri is compared with the period-colour relation of Hipparcos data (field W UMa type) and contact binaries present in other globular clusters.