The symbiotic star YY Herculis. I. Photometric history over 1890-1996.

The lightcurve of the symbiotic star YY Her extending from 1890 to 1996 is presented and discussed. YY Her underwent four outbursts in the last 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 thermonuclear flashes with no mass loss induced by super-Eddington luminosities. An orbital period of 590 days is inferred from the quiescence {DELTA}V=0.3mag modulation ascribed to a heating effect. Four enigmatic deep minima in 1952, 1965, 1971 and 1992 appeared at the right phase for an eclipse to occur, but such an explanation encounters severe difficulties. At least two of these minima occurred right at the onset of an active phase or outburst and this suggests an alternative interpretation to the eclipse hypothesis. YY Her appears to belong to the Bulge/thick-Disk population of the Galaxy, with D=3.2kpc, z=950pc and E_B-V_=0.41