The period of the globular cluster X-ray source X 1820-303; confirmation from Ariel V data.

The authors have analysed archival Ariel V data to search for the proposed period of X 1820-303. A modulation with a period of 685.02±0.54 s (2σ confidence) and a peak-to-peak amplitude of 2.2 per cent is detected from the longest data set. Combining this measurement with the EXOSAT results the authors find the upper limit on the period derivative to be 0.28 s yr^-1 (90 per cent confidence). The stability of the period over a time-scale of 10 years suggests that the period is orbital in nature.