The Closest of the Close: Observational and Modeling Progress

Two of today's most critical unresolved problems in binary star research are the formation and structure of W Ursae Majoris binaries. Demands imposed by nearly equal effective temperatures for the two stars lead to a grossly different mass–luminosity relation from that of single main sequence stars. That only non-equilibrium models can account for many W UMa properties is widely recognized. One such model is the thermal relaxation oscillator model, in which systems oscillate between states of good thermal contact and broken contact. Notwithstanding decades of theoretical research, W UMa's have not been fitted into a comprehensive structural theory, and the formation mechanism also needs to be resolved. The recent idea that all W UMa's are part of triple (or multiple) systems and formed through dynamical interactions with third bodies may well answer that question. On the observational front, we have seen in the last decade a vast increase in the number of doublelined systems with complete radial velocity curves that leads to parameters of improved reliability, including absolute dimensions. After a brief review of observational efforts, we look at recent modeling progress, describe a project aimed at determining broken-contact binary parameters, and comment on specific challenges with W Crv as an example. We then discuss finding third light from simultaneous light and velocity solutions for W UMa's suspected of having tertiary components. Showing the specific example of V2388 Oph, we explore the prospects of this method for finding W UMa companions.