The ongoing 2008-09 outburst of CI Cyg

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 BV R(C) I(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 Delta B = 1.9, Delta V = 1.5, Delta R(C) = 0.9, Delta I(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) similar to 6900 K and R = 28 R(circle dot). The high-ionization emission lines ([Ne V], [Fe VII], He II) disappeared and only lower ionization lines were visible. Balmer and He I 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 2 x 10(-8) M(circle dot) yr(-1) rate.