A bit of a further update:
In terms of the light curve, we've had increasingly rapid oscillations since around 6 Jan, with an amplitude of 0.5-1 mag and a period of 1-1.5 days. *Possibly* these are smoothing out recently into a steadier decline, although I'm not betting on it.
The first image below shows the spectra from one of the maxima (8 Jan, V=c.4.5) and one of the minima (V=c.5.1). The lines visible in the two spectra are essentially the same - but the relative intensity of the Balmer lines (especially Ha) is much greater at minimum. These are normalised spectra, not flux calibrated - in other words, we're comparing relative intensity of spectral features, not absolute intensity. So at minimum, the emission lines are relatively stronger compared to the stellar continuum. This lends support to the theory that these oscillations are a product of changes in the optical thickness of the ejecta. As the ejecta fireball becomes optically thin, we see a greater relative intensity of the ionised gas shell (emission lines) and of the UV/FUV radiation from the white dwarf itself. Well, we don't actually "see" the latter, which is why the visual-wavelength magnitude drops.
In the last week, everything has begun to change. The second attachment compares spectra from 17-25 Jan. All the forbidden lines are growing in intensity while the "iron curtain" of FeII emisison lines are beginning to fade in relative intensity. Some other significant changes (marked with triangles):
1) [OIII] at 4363 has emerged - although in my low-resolution spectrum this merges with H gamma at 4340. Good to get a higher-res picture of this area.
2) An entirely new line has emerged around 4670-80. Is this He II? That's not implausible
3) OI at 8446 has gone nuts. According to one theory (e.g. Williams, 2012), this is a product not of the white dwarf emission itself, but of interaction with the material coming off the secondary star.
I won't begin to speculate as to the astrophysical causes. But the spectra and photometry at least suggest we may be entering the transition phase of the nova.
Cheers
Jonathan
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