2025MNRAS.541.2553K

The CM Draconis system is a well-studied, double-lined spectroscopic binary that is totally eclipsing and exhibits strong magnetic activity. Nearly one million photometric measurements have been collected across multiple wavelengths over more than half a century. In addition to showing frequent flare activity and apsidal motion, CM Dra also hosts a distant white dwarf and has been proposed to harbour a Jupiter-sized circumbinary companion. At only 47 light-years from Earth, it remains one of the most observationally rich and dynamically intriguing low-mass binary systems. We present a comprehensive photometric and spectroscopic analysis of the system using new ground-based observations and data from 19 sectors of the TESS mission. We derive precise fundamental parameters for both components: $M_1 = 0.2307 \pm 0.0008\, {\rm M}_\odot$, $M_2 = 0.2136 \pm 0.0008\, {\rm M}_\odot$, $R_1 = 0.2638 \pm 0.0011\, {\rm R}_\odot$, $R_2 = 0.2458 \pm 0.0010\, {\rm R}_\odot$, $L_1 = 0.0060 \pm 0.0005\, {\rm L}_\odot$, and $L_2 = 0.0050 \pm 0.0004\, {\rm L}_\odot$. The derived distance ($14.4 \pm 0.6$ pc) is consistent with Gaia DR3 measurements. Eclipse timing variations (ETVs) spanning over five decades were analysed in detail. A long-period ($\sim$56 yr) modulation was identified, which may be attributed either to the light-time effect of a possible circumbinary companion or to magnetic activity cycles. While the Bayesian Information Criterion statistically favours the model involving a light-time effect from a planetary companion, stellar activity remains a viable alternative that cannot yet be ruled out. Our results demonstrate that CM Dra is a valuable test case for studying both stellar activity and the potential presence of circumbinary companions in multiple-star systems. Continued long-term monitoring will be essential to distinguish between these competing scenarios.