Astron. Astrophys. 330, 533-540 (1998)

1. Introduction

Over the last two decades the eclipsing binary DI Her has been object of intensive investigations due to the discrepancy between the observed apsidal motion rate and the theoretical predictions. Discrepancies between observed and theoretical rates were found in some eclipsing binary systems which exhibit apsidal motion indicating that real stars seem to be more mass concentrated than predicted by theoretical models. Such disagreements were investigated by Claret & Giménez (1993a,b). For non-relativistic systems the improvements introduced in stellar evolution model such as new opacities, mass loss, core overshooting and rotation, were enough to reduce the differences between observation and theory down to acceptable levels. The new input physics contributed to decrease the theoretical values of k₂ , i.e, the new models are more centrally condensed in mass than the previous ones. Of course, the high quality of the observational data were of prime importance to obtain such an agreement. For a detailed discussion see the papers by Claret & Giménez (1993a,b). Recent investigation by Claret (1997) indicates that also the relativistic systems, for which good absolute dimensions determination are available, compare very well with stellar models and the prediction by the General Relativity (GR).

However, such improvements were not sufficient to explain the case of DI Her. The observed apsidal motion rate is too slow when compared with the theoretical predictions. Several hypothesis were put forward in order to explain such a disagreement. For example, Guinan & Maloney 1985 have considered four possibilities:

1. A rapid circularization of the orbit
2. The presence of a circumstellar cloud between the components
3. The presence of a third star
4. Use of an alternative theory of gravitation.

On the other hand Shakura (1984) suggested that the misalignment of the orbital and spin angular momenta could explain the anomalous periastron shift and Company et al. (1987) explored in more detail this possibility.

These and other explanations for the problem of DI Her are revised in this paper making use of new observational data as well as modern stellar models. New possible alternatives also introduced.

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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