13 Ceti

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13 Ceti is a triple-star system in the constellation Cetus. It is barely visible to the naked eye, with a combined brightness of about magnitude 5.2, and lies about 69 light-years away. The system is moving away from us at roughly 10 km/s and may share a common motion with the Hyades group, though it is older than that group.

System layout
- Outer pair: The bright star A forms a wide pair with star B. They orbit each other every about 6.9 years. Star A is the brighter member (visual magnitude ~5.6) and is an F-type main-sequence star (F6 V). Star B is a sun-like G-type main-sequence star (G4 V) with magnitude ~6.9.
- Inner binary: The star A component is itself a close binary (Aa and Ab) with a 2.08-day orbit. Aa has about 1.18 solar masses and Ab about 0.35 solar masses; the orbit is nearly circular.
- A distant companion: A third star, C, is a background object of magnitude ~12.5 located about 24 arcseconds away.

Notes on activity and age
- The brighter star A shows signs of chromospheric activity and is classified as an RS Canum Venaticorum variable, BU Cet.
- The system’s stars have ages around a few billion years, with the individual components estimated between about 2.5 and 3.8 billion years old.

Key data (simplified)
- Distance: about 69 light-years (37 parsecs? in this system’s measurements, 21.3 pc derived from parallax)
- Radial velocity: +10.4 km/s
- Magnitudes: A ~5.6, B ~6.9, C ~12.5
- Spectral types: Aa (F6 V), Ab (K3.5 V), B (G4 V)
- Outer orbit: period ~6.9 years, eccentricity ~0.76
- Inner orbit: period ~2.08 days, eccentricity ~0 (circular)