2008IAUC.8938....2R

R. J. Rudy, D. K. Lynch, and R. W. Russell, The Aerospace Corporation; C. E. Woodward, University of Minnesota; and K. Covey, Center for Astrophysics, report on SpeX observations (wavelength range 0.8-2.5 microns) of this nova (cf. IAUC 8934) obtained at the Infrared Telescope Facility on Apr. 17.6 UT. In the five days since Lynch et al. (IAUC 8935) observed V2491 Cyg on Apr. 12, it declined in brightness by a factor of 3. The strengths of the emission lines compared to the continuum have generally increased, as has the excitation of the emission-line spectrum. The neutral helium triplet and singlet at 1.08 and 2.06 microns, respectively, doubled in size relative to the hydrogen lines. Although the C I and N I lines decreased, the O I lines at 0.84 and 1.13 microns, which are fluorescently excited by Lyman_beta, are now the strongest emission features in the infrared spectrum. Their increase has permitted more accurate measurements of their relative strengths, which indicate a reddening of E(B-V) = 0.43, a value slightly greater than that reported by Lynch et al. Photometric magnitudes on Apr. 17: J = 7.7, H = 7.8, K = 7.4.