Variability class · ROT · RS · BY

Rotational

Starspots carried across the disk by rotation.

Rotational variables aren't pulsating or eclipsing — they're spotted. Magnetic active regions, darker or brighter than the surrounding photosphere, sweep in and out of view as the star turns, modulating the light at the rotation period.

158,902 active stars ROT · RS · BY

Phase φ 0.00 · Δm 0.00

A low-amplitude, quasi-sinusoidal modulation — often with two unequal minima per cycle when two active longitudes face us in turn.

What it is

A magnetically active star — a spotted dwarf, an active binary, or a chemically peculiar star with abundance patches — whose surface brightness is uneven. As it rotates, the visible mix of bright and dark regions changes smoothly, so the period you recover is the rotation period itself (hours for rapid rotators, weeks for slow ones).

The physics

Magnetic fields suppress convection and create cool starspots (or, in Ap/Bp stars, chemically anomalous bright spots). The modulation amplitude tracks how spotted the star is and where the spots sit. In the RS CVn active binaries, tidal interaction spins the stars up and keeps them perpetually active; ellipsoidal variables (ELL) are a borderline case where tidal distortion, not spots, does the modulating.

The families within

Subtypes

Spotted star

ROT

Generic spotted rotator; low-amplitude, quasi-sinusoidal modulation at the rotation period.

RS CVn

Chromospherically active close binary — large spots, strong activity, often with eclipses too.

BY Draconis

Spotted, emission-line K–M dwarfs; the low-mass end of rotational variability.

The varchive method

Finding the period

The same science code runs for every star. Here is how it behaves for this class — and where it can be fooled.

Recovered by GLS, but the amplitudes are small, so camera conditioning is essential — an uncorrected ~0.3 mag camera zero-point step would otherwise alias as a spurious ~1-day signal and drown a real rotation peak. Because two active longitudes can produce two minima per turn, there is a genuine period-vs-half-period ambiguity to resolve, much like the EW doubling.

What to watch for

Spots emerge, migrate, and decay over weeks, so the fold blurs over a long baseline — these stars are only quasi-periodic.
Two spot groups can mimic a star at half the true rotation period (the P vs P/2 question again).
Without per-camera conditioning, instrumental steps masquerade as ~1-day rotation signals.
Ellipsoidal variables hide here too — their photometric period is half the orbital period, since the tidal bulge faces us twice per orbit.

From the archive

Worked examples

ASASSN-V J202758.57−792847.3

ROT · a 3.6-day spotted rotator

#2980 →

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Browse the Rotational population

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