2009MNRAS.399.2139S

In this paper, we discuss the early phases of the ongoing outburst that CI Cyg, a prototype symbiotic star, is currently undergoing after 30-year quiescence. We have tightly monitored CI Cyg in B V R_CI_C bands, starting a whole year before the onset of the outburst, and in addition we obtained numerous Echelle high- and low-resolution absolutely flux-calibrated spectra. The outburst started while the accreting white dwarf (WD) was being eclipsed by the Roche lobe filling M giant companion, and it was discovered during the egress phase on the second half of 2008 August. The system reached peak V-band brightness in early 2008 October and has been characterized by amplitudes ΔB = 1.9, ΔV = 1.5, ΔR_C = 0.9, ΔI_C= 0.4 mag. At maximum V-band brightness, the outbursting WD had expanded to closely resemble an F3 II/Ib star, with M_V = -3.5, T_eff∼ 6900 K and R = 28R_☉. The high-ionization emission lines ([Nev], [Fevii], Heii) disappeared and only lower ionization lines were visible. Balmer and Hei emission lines declined in equivalent width but increased in absolute flux. The output radiated by the hot component during the outburst corresponds to nuclear burning proceeding at a 2x10^–8M_☉/yr rate.