Photometric and spectrophotometric observations of the classical symbiotic star YY Her during its return to quiescence

Abstract

We present new photometric UBVRI and spectroscopic observations of the symbiotic star YY Her during its return to quiescence after a strong outburst in 1993. High-resolution spectra of YY Her at similar phases at outburst maximum ϕ=0.48) and in quiescence ϕ=0.37) are presented for the first time. The ephemeris of YY Her has been refined P=586d). The last two observed minima (in 1999 and 2000) differed radically in shape from the 1997 minimum described previously. Both were sharp and deep $$(\Delta U \sim 1\mathop .\limits^m 6, \Delta V \sim 0\mathop .\limits^m 9)$$ . To explain this shape of the V light curve, which is only slightly affected by nebular emission, it should be assumed that the cool component of YY Her fills much of its Roche lobe and has a hot spot on the hemisphere facing the hot component. The emission spectrum rich in Fe II lines, which is characteristic of symbiotic stars, was observed during the outburst, but high-ionization lines (He II λ4686) were also observed. The He I λλ5876, 7065 lines exhibit distinct P Cyg profiles; the centers of the absorption components are shifted from the emission ones by V r≈100 km s−1, suggesting moderate outflow velocities.