No unique quadratic ephemeris for VW Cep

TL;DR

This study analyzes the orbital period variations of VW Cep, revealing multiple reversals in mass transfer direction over nearly a century, challenging the notion of a unique quadratic ephemeris for this binary system.

Contribution

It introduces a model allowing for multiple mass transfer reversals in VW Cep, explaining observed discontinuities in its eclipse timing data.

Findings

Period increase from 1926 to 1943 due to mass transfer from less massive star

Reversal of mass transfer direction after 1946, leading to period decrease

Recent period increase in 1999 indicating another reversal in mass transfer

Abstract

From the eclipsing binary VW Cep, 1963 epochs of primary and secondary minima have been registered since 1926. Because VW Cep is a shallow contact binary the direction of the flow from one component to the other could possibly be reversed. We aim to fit the observed eclipses into a scenario in which reversals may occur. Orbital period changes can not be fitted into a unique quadratic ephemeris, because of discontinuities in the (O-C) vs E graph. We try to obtain better agreement through the allowance of RLOF reversal in this binary system. From 1926 until 1943 the orbital period increase fits into a model of mass transfer from the less massive component. From 1946 on the direction of the mass transfer changed so that the most massive component became the donor star, as shown by a systematic period decrease. A sudden period increase in 1999 introduces a currently underway episode of mass transfer from the less massive component. The ephemeris of VW Cep can be fitted into a model in which the direction of the mass transfer was reversed twice since 1926.