# Long-term Spot Stability in the Post-common-envelope Binary QS Vir

## TL;DR

This study presents a comprehensive 25-year photometric analysis of QS Vir, revealing a stable dark spot on the red dwarf component that migrated between 1993 and 2002, indicating long-term magnetic activity stability.

## Contribution

First complete photometric modeling of QS Vir using extensive multi-year data, demonstrating long-term stability of a magnetic spot on the red dwarf component.

## Key findings

- Dark spot responsible for light curve asymmetry has been stable for 15 years.
- A single spot model cannot fit all filter data simultaneously, indicating complex surface activity.
- Spot migration between 1993 and 2002 suggests a flip-flop magnetic activity event.

## Abstract

We observed the post-common-envelope eclipsing binary with a white dwarf component, QS Vir, using the 1.88 m telescope of Kotammia Observatory in Egypt. The new observations were analyzed together with all multicolor light curves available online (sampling a period of 25 yr), using a full-feature binary system modeling software based on Roche geometry. This is the first time complete photometric modeling was done with most of these data. QS Vir is a detached system, with the red dwarf component underfilling its Roche lobe by a small margin. All light curves feature out-of-eclipse variability that is associated with ellipsoidal variation, mutual irradiation and irregularities in surface brightness of the tidally distorted and magnetically active red dwarf. We tested models with one, two, and three dark spots and found that one spot is sufficient to account for the light curve asymmetry in all data sets, although this does not rule out the presence of multiple spots. We also found that a single spotted model cannot fit light curves observed simultaneously in different filters. Instead, each filter requires a different spot configuration. To thoroughly explore the parameter space of spot locations, we devised a grid-search procedure and used it to find consistent solutions. Based on this, we conclude that the dark spot responsible for light curve distortions has been stable for the past 15 yr, after a major migration that happened between 1993 and 2002, possibly due to a flip-flop event.