Near-infrared studies during maximum and early decline of Nova Cephei 2014 and Nova Scorpii 2015

We present multiepoch near-infrared photospectroscopic observations of Nova Cephei 2014 and Nova Scorpii 2015, discovered in outburst on 2014 March 8.79 UT and 2015 February 11.84 UT, respectively. Nova Cep 2014 shows the conventional NIR characteristics of a Fe II class nova characterized by strong C I, HI and OI lines, whereas Nova Sco 2015 is shown to belong to the He/N class with strong He I, HI and OI emission lines. The highlight of the results consists in demonstrating that Nova Sco 2015 is a symbiotic system containing a giant secondary. Leaving aside the T CrB class of recurrent novae, all of which have giant donors, Nova Sco 2015 is shown to be only the third classical nova to be found with a giant secondary. The evidence for the symbiotic nature is threefold; first is the presence of a strong decelerative shock accompanying the passage of the nova's ejecta through the giant's wind, second is the H a excess seen from the system and third is the spectral energy distribution of the secondary in quiescence typical of a cool late-type giant. The evolution of the strength and shape of the emission line profiles shows that the ejecta velocity follows a power-law decay with time (t^-1.13±0.17). A Case B recombination analysis of the HI Brackett lines shows that these lines are affected by optical depth effects for both the novae. Using this analysis, we make estimates for both the novae of the emission measure n_e²L, the electron density n_e and the mass of the ejecta.