THE SYMBIOTIC STAR YY HERCULIS .1. PHOTOMETRIC HISTORY OVER 1890-1996

The lightcurve of the symbiotic star YY Her extending from 1890 to 199 6 is presented and discussed. YY Her underwent four outbursts in the l ast century, in 1914-19, 1930-33, 1981-82 and 1993-96, with additional bright phases in 1890, 1903, 1942, 1954, 1965 and 1974. The absolute magnitude reached at peak brightness is adequate for mild thermonuclea r flashes with no mass loss induced by super-Eddington luminosities. A n orbital period of 590 days is inferred from the quiescence Delta V=0 .3 mag modulation ascribed to a heating effect. Four enigmatic deep mi nima in 1952, 1965, 1971 and 1992 appeared at the right phase for an e clipse to occur, but such an explanation encounters severe difficultie s. At least two of these minima occurred right at the onset of an acti ve phase or outburst and this suggests an alternative interpretation t o the eclipse hypothesis. YY Her appears to belong to the Bulge/thick- Disk population of the Galaxy, with D = 3.2 kpc, z = 950 pc and EB-v=0 .41.