A superhump period is measured from time-series photometry during a dwarf-nova event. The practical challenge is to separate the changing modulation from the overall outburst, daily sampling aliases and the binary’s orbital clock. A strong periodogram peak is the beginning of the analysis, not its final answer.
Preserve the information before searching
Calibrate the images, use suitable comparison and check stars, and report the passband and uncertainty. Record exposure midpoint times and state both the time reference and timescale. JD, HJD and BJD are not interchangeable labels, and a barycentric correction does not by itself specify UTC versus TDB.
The AAVSO observing manuals provide practical photometry guidance. Combining stations requires checking zero-point differences and clock conventions before interpreting small timing drifts.
Find candidate periods and test aliases
Remove only a sufficiently slow outburst trend. Aggressive detrending can absorb genuine modulation. A Lomb–Scargle or phase-dispersion search identifies candidate periods, but gaps between nights can produce competing aliases.
Fold the data on each plausible period and inspect individual nights as well as the combined curve. Do not select an alias merely because it produces the most attractive plot. Longer coverage and overlapping stations are often more decisive than adding decimal places to one short run’s best period.
Time maxima and keep a cycle count
Measure each reliable superhump maximum with a suitable local fit or template. Assign cycle number E and compare with Tcalc = T0 + E P0. The residual O − C shows whether maxima drift relative to that reference clock.
A cycle-count error across a gap can create an apparent period jump. Changes in waveform can move a fitted maximum even without the same change in physical precession. Publish the maxima, uncertainties and cycle assignments so another reader can reproduce the result.
Calculate period excess correctly
For positive superhumps, the period excess is ε = (Psh − Porb)/Porb. If an independently measured orbital period is 0.080 days and a superhump period is 0.081 days, ε = 0.0125, or 1.25%.
The frequency-form excess is different: ε* = 1 − Porb/Psh, about 1.235% in this example. The quantities are close, but should not be substituted silently. Neither value alone is a model-free mass ratio.
Do not average away the evolution
Growing stage A, evolving stage B and later stage C superhumps can have different periods. Stage identification needs adequate temporal coverage. The Kato and Osaki stage-A method estimates mass ratio under a specific dynamical-precession interpretation, rather than treating every superhump period with one universal calibration.
The multi-system survey of superhump period variations illustrates why timing and evolutionary stage are reported together.
For the underlying mechanism, see SU UMa superoutbursts. Compare orbital period changes to keep the disk clock separate from the binary clock.
Revised October 26, 2026. Period-excess numbers are illustrative.