Detection of a approximately 78 day Period in the RXTE, VELA 5B, and Ariel 5 All-Sky Monitor Data of Cygnus X-2

We report the detection of a 77.7 +/- 1.0 day modulation in the X-ray light curve of Cygnus X-2, obtained by the all-sky monitor on board the Rossi X-ray Timing Explorer. The modulation has an amplitude of ~40%. A secondary minimum is present with an amplitude of ~20%. The hardness of the X-ray spectrum as a function of this modulation is anticorrelated with the source intensity. Reanalysis of archival data from Vela 5B and Ariel 5 shows independent confirmation of this periodicity, with significant periods of 77.4 +/- 0.2 and 77.7 +/- 0.2 days, respectively. Using these three data sets we determine a single combined long-term ephemeris of JD 244,2209.0 +/- 4.7 + N x (77.79 +/- 0.08), where phase zero is defined as the primary minimum. The occurrence of the different intensity states found in pointed observations with the Einstein, EXOSAT, and Ginga satellites is also consistent with this ephemeris. The 78-day periodicity is therefore a reliable clock in the system, and we tentatively associate this period with the precession of a tilted accretion disk, similar to those seen in some massive X-ray binaries and Her X-1.