DETECTION OF A APPROXIMATE-TO-78-DAY PERIOD IN THE RXTE, VELA-5B, ANDARIEL-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 li ght 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 simila r to 40%. A secondary minimum is present with an amplitude of similar to 20%. The hardness of the X-ray spectrum as a function of this modul ation is anticorrelated with the source intensity. Reanalysis of archi val data from Vela 5B and Ariel 5 shows independent confirmation of th is periodicity with significant periods of 77.4+/-0.2 and 77.7+/-0.2 d ays, respectively. Using these three data sets we determine a single c ombined long-term ephemeris of JD 244,2209.0+/-4.7+Nx(77.79+/-0.08), w here phase zero is defined as the primary minimum. The occurrence of t he different intensity states found in pointed observations with the E instein, EXOSAT, and Ginga satellites is also consistent with this eph emeris. The 78-day periodicity is therefore a reliable clock in the sy stem, and we tentatively associate this period with the precession of a tilted accretion disk, similar to those seen in some massive X-ray b inaries and Her X-1.