# Origin of the orbital period change in contact binary stars

## TL;DR

This paper explains the orbital period changes in contact binary stars as a result of mass asymmetry evolution towards symmetry, involving energy conversion and implications for stellar mergers.

## Contribution

It introduces a model linking orbital period changes to mass asymmetry evolution and predicts stages of symmetrization in contact binary stars.

## Key findings

- Orbital period variations correlate with stages of binary symmetrization.
- Significant energy (~10^41 J) is released during symmetrization.
- Star mergers are energetically unfavorable according to the model.

## Abstract

The evolution of contact binary star systems in mass asymmetry (transfer) coordinate is considered. The orbital period changes is explained by an evolution in mass asymmetry towards the symmetry (symmetrization of binary system). It is predicted that a decreasing and an increasing orbital periods are related, respectively, with the non-overlapping and overlapping stage of the binary star during its symmetrization. A huge amount of energy $\Delta U\approx 10^{41}$ J is converted from the potential energy into internal energy of the stars during the symmetrization. As shown, the merger of stars in the binary systems, including KIC 9832227, is energetically an unfavorable process. The sensitivity of the calculated results to the values of total mass and orbital angular momentum is analyzed.