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We present near-infrared (1-2.5 micron) JHK photo-spectroscopic results ofthe unusually slow nova V5558 Sgr (2007). V5558 Sgr showed a slow climb tomaximum that lasted for about 60 days and then underwent at least five strongsecondary outbursts. We have analyzed the optical light curve to derive larget2 and t3 values of 281 +/- 3 and 473 +/- 3 days respectively. An alternateapproach is adopted to derive a distance estimate of 1.55 +/- 0.25 kpc asconventional MMRD relation may not be applicable for a slow nova. In thepre-maxima stage the spectra showed narrow (FWHM ~ 400 - 550 km/s and strongemission lines of Paschen and Brackett series with prominent P-Cygnicomponents. In the later phase the spectra show significant changes with thedevelopment of strong and broad ~ 1000 km/s emission lines of HI, HeI, OI, andNI and some uncommon Fe II emission lines. No evidence of dust formation isseen. V5558 Sgr has been shown to be a rare hybrid nova showing a transitionfrom He/N to Fe II type from optical spectra. However the near-infrared data donot show such a transition and we discuss this anomalous behavior. Arecombination analysis of the Brackett lines allows us to constrain theelectron density and emission measure during the early optically thick phaseand to estimate the mass of the ejecta to be (6.0 +/- 1.5) x 10^(-4) Msun,assuming a filling factor of unity, from later observations.

Near-infrared studies of V5558 Sgr: an unusually slow nova with multiple outbursts