CM Draconis Binary: Low-Mass Stellar Benchmark

Updated 27 July 2025

CM Draconis is a nearby, fully convective, double-lined eclipsing binary composed of nearly identical M4.5 dwarf stars with deep eclipses and precisely measured fundamental parameters.
Extensive TESS photometry and spectroscopic analyses yield subpercent-level mass, radius, and orbital measurements while revealing strong magnetic activity, periodic flares, and evolving spot cycles.
Long-term eclipse timing variations indicate either a circumbinary planet or magnetic cycle effects, establishing CM Draconis as a critical benchmark for refining low-mass stellar models.

The CM Draconis (CM Dra) system is a nearby, low-mass, double-lined spectroscopic binary composed of two nearly identical fully convective M4.5 dwarf stars in a short-period, totally eclipsing orbit. Located at approximately 14.4 ± 0.6 pc (corresponding to Gaia DR3 parallax of 67.29 ± 0.03 mas), it is dynamically rich and observationally accessible, making it an archetypal benchmark for testing models of stellar structure, evolution, magnetic activity, and multiplicity in low-mass stars (Kalomeni et al., 22 Jul 2025). The system also includes a distant white dwarf companion (WD 1633+572) and presents compelling evidence for both strong magnetic phenomena and potential circumbinary planetary companions.

1. Fundamental Stellar and Orbital Properties

Extensive photometric and spectroscopic campaigns—most recently including 19 sectors of Transiting Exoplanet Survey Satellite (TESS) data—have yielded highly precise fundamental parameters for the CM Dra components. The most robust recent determinations give (Kalomeni et al., 22 Jul 2025):

Star	$M$ ( $M_\odot$ )	$R$ ( $R_\odot$ )	$L$ ( $L_\odot$ )
Primary	0.2307 ± 0.0008	0.2638 ± 0.0011	0.0060 ± 0.0005
Secondary	0.2136 ± 0.0008	0.2458 ± 0.0010	0.0050 ± 0.0004

These values are consistent within uncertainties with earlier determinations, which showed mass and radius uncertainties at the subpercent level (Martin et al., 2023, 1106.1452). The orbital period is P ≃ 1.268 days, and the binary is almost perfectly edge-on, yielding deep, total eclipses that facilitate stringent determinations of system geometry and component dimensions. The derived distance is in full agreement with Gaia astrometry.

2. Magnetic Activity: Spots, Flares, and Cycles

CM Dra is characterized by strong and persistent magnetic activity. Both photometric and spectroscopic observations reveal pronounced and variable spot modulation, with light-curve analyses indicating spot-induced periodicities at the orbital period and at half this value (Martin et al., 2023). This is consistent with either two dominant, longitude-separated spot clusters on a single star, or synchronized spot clusters near the substellar points on both stars.

A key finding is the detection of a possible activity cycle of order ≈ 4 years, inferred from evolving spot amplitudes and phase drifts in the TESS light curves (Martin et al., 2023). Flare activity is also prominent: a neural network–aided search over 15 TESS sectors reveals 163 flares, with an average rate of ~0.5 flares per day for the binary (thus ~0.25 per star). Flare locations are likely preferentially polar, as evidenced by the non-reduced flare rate during primary and secondary eclipses—a consequence of the near–90° inclination. The flare occurrence rate is positively correlated with phases of spot modulation where the system appears brighter, suggesting association with bright magnetic features (plages) as well as dark spots.

3. Metallicity, Age, and Chemical Composition

The metallicity of CM Dra has been the subject of detailed paper. Near-infrared spectra obtained with the SpeX instrument at IRTF, analyzed using empirical H- and K-band calibrations, yield [Fe/H] = –0.30 ± 0.12, robust across orbital phases (1210.4736). This measurement is critical for constraining input parameters for stellar models. However, the system’s kinematics (high space velocities: U, V, W ≈ 105, –120, –36 km s⁻¹), combined with the properties of the white dwarf companion, identify CM Dra as a thick disk member with a formal age of 8.5 ± 3.5 Gyr (Feiden et al., 2014). Thick disk stars are α-enhanced; when this is accounted for, previous [Fe/H] estimates may be underestimates by ~0.1–0.3 dex, and a value near –0.1 may be more appropriate.

The elevated age and likely α–enhancement lessen, but do not eliminate, the well-known mass–radius discrepancy for low-mass stars: standard models underpredict the observed radii by only ~2–3% when these revised inputs are used, reduced from earlier discrepancies of 6–7% (Feiden et al., 2014).

4. Magnetic Components and Theoretical Modeling

Magnetic fields in CM Dra have multiple observational and theoretical consequences:

Convection inhibition: Magnetic fields reduce convective efficiency, a process formalized using the Gough–Tayler (GT) criterion: ∇ > ∇ₐ𝑑 + δ, with δ = B²/(4πγP). This leads to larger radii and lower luminosities than in non-magnetic models (1106.1452).
Starspots: High-latitude spot features, traced via ~1–3% amplitude sinusoidal variations in the light curve, not only reduce emergent flux but can bias radius measurements derived from eclipse photometry. Modeling corrections for spot coverage indicate the true radii are a few percent smaller than naively measured.

Full evolutionary models including magnetic inhibition (both constant-δ and “ceiling” field profiles) and spot corrections yield predictions that can match the observed radii and luminosities for CM Dra at an age of ~4–9 Gyr. Near-surface vertical (poloidal) fields of ~460–510 G are indicated. In “ceiling” models, interior fields are capped at ~1 MG, avoiding physically implausible field strengths (>10⁷ G).

Evolutionary calculations use a relaxation method, solar-calibrated or adjusted mixing length (α), and detailed equations of state. Sensitivity derivatives of log L and log R to α, Y, and Z quantify the effects of parameter variations.

5. Eclipse Timing Variations and System Architecture

An exceptionally long baseline of eclipse timing measurements (over 50 years, comprising 307 minima) reveals eclipse timing variations (ETVs) with a sinusoidal modulation on ~56-year timescales (Kalomeni et al., 22 Jul 2025). Two competing hypotheses are considered:

Light-time effect (LTTE): The periodic O–C signal is consistent with the gravitational influence of a circumbinary companion (minimum mass ~1.2 M_J at edge-on), inducing the reflex motion of the binary. The combined apsidal motion + LTTE model is statistically favored according to the Bayesian Information Criterion (ΔBIC ≈ 39) over an apsidal-motion-only model.
Magnetic activity cycles: Quasi-periodic ETVs may be caused by shifts induced by surface spot cycles or other magnetic phenomena. These can mimic LTTE effects, and such activity-induced ETVs have been documented in other low-mass binaries.

The ambiguity persists: while the statistical tests favor a circumbinary planet, magnetic activity cannot be ruled out. Continued long-term photometric and spectroscopic monitoring is required to distinguish between these hypotheses.

6. Broader Implications and Future Directions

CM Dra is a critical benchmark for the calibration and testing of stellar evolution and magnetic models in the low-mass, fully convective star regime. The system offers the following:

Tests of magnetic models: The system’s radii and luminosities are consistent with models only when magnetic inhibition of convection and surface spot corrections are accounted for.
Precision parameters: Recent TESS-based studies have reduced measurement uncertainties on fundamental parameters to below 0.1%, enabling stringent model constraints (Martin et al., 2023, Kalomeni et al., 22 Jul 2025).
Multiplicities and planet formation: With evidence for both a wide white dwarf companion and a possible Jupiter-mass circumbinary planet, CM Dra also serves as a laboratory for the dynamics and formation of hierarchical stellar and planetary systems.
Space weather and habitability: The inferred polar flare distribution may moderate the direct exposure of equatorial planets to high-energy flare events, which is relevant for exoplanet habitability studies.
Model–observation convergence: Application of updated age and composition constraints has substantially reduced, though not fully eliminated, the radius inflation discrepancies, suggesting that further progress will rely on improved understanding of magnetic topology, spot coverage, and α–element abundance patterns.

In summary, CM Draconis, through its precise measurements, magnetic phenomena, and long-term monitoring, continues to underpin the refinement of models for fully convective low-mass stars, the interpretation of magnetic activity signatures, and the exploration of planet formation and habitability in tightly bound binary systems.