ph}$, and photospheric radius $R_{\rm ph}$, that is, $F_\nu \propto \dot M^2_{\rm wind} / v^2_{\rm ph} R_{\rm ph}$ [@hac06kb], where $F_\nu$ is the flux of free-free emission at the frequency $\nu$. It should be noted that the flux of free-free emission $F_\nu$ is almost independent of the frequency $\nu$ in the optical and near-infrared (NIR) region.

@hac06kb calculated free-free emission light curves for various WD masses (from $0.5~M_\sun$ to $1.38~M_\sun$) and envelope chemical compositions. Since the flux of free-free emission $F_\nu$ is almost independent of the frequency $\nu$, the light curve shapes are very similar among the optical and NIR bands until the nebular phase or dust formation started. For example, the $V$, $J$, $H$, $2.3\mu$m, and $3.6 \mu$m band light curves of V1668 Cyg show an almost identical light curve shape, which are well reproduced by a theoretical light curve until the optically-thin dust shell formation makes a deviation from the theoretical one [see, e.g., Figure 18 of @hac06kb].

@hac06kb further found that the theoretical free-free emission light curves and UV (1455 Å) light curves show a scaling law among various WD masses and chemical compositions. The UV 1455Å  band is a narrow (1445–1465Å) and emission line-free band invented 